This is in no way a definitive guide on how you should play, it is just a few observations put together to possibly help some of the new players on this co-op mode. Ultimately, play how you want. I wish I could provide some images but unfortunately this will have to do.
Some quick general facts and tips:
Yes, you can get the Beserker by wave 7. If there are any wave restarts (Host quitting) it is possible to get it at wave 5&6. This entirely depends on the map, COG spawn locations and how well your team is doing compared to your individual contribution. More on that later.
Bloodmount Beast riders will shoot at Decoys when in range.
Avoid jumping in front of a turret only to get to a downed Hero. it doesn't make any sense to do that.
When Anya or Carmine drop the mortar and shield (respectively) they cannot get it back. This either happens by downing them or in Anya's case you come too close to her and she is in cover. She will hardly drop the mortar mid-stance and will even try to blind fire at close range.
Onyx Guard with One Shot will never drop his weapon. Good thing is he is the only one that won't barrel-roll wave 12.
Last enemy is usually designated with a small blue triangle above its head. This can be seen without the TAC/COM (left bumper) depressed.
TAC/COM is a good way to see where the Silverback is going even when it is behind cover. It is the only Fortification health bar that moves.
All Flame grenades, Flame turrets and HOD will stun the Beserker. You must learn to time the start of your charge near the end of the stun animation. Flame turrets may take two Berserker charges because of their range. If you are charging a Turret, try to avoid running into a Hero as it will slow you down even further.
Clayton Carmine will sometimes go totally off course and be left as the last man standing. The blue triangle does not always show for him when he is with the shield.
If you see a Silverback, Spot it and shoot. These are absolutely lethal, especially in rocket mode.
Bloodmounts cannot melee/claw when sitting directly over the downed enemy. Frustrating I know. Even if they are just off to the side, you can end up missing them
Berserkers cannot be healed by Kantus or Armored Kantus
Berserkers need to earn roughly $2200 per wave to be sustainable. You can do this strictly by barriers,turrets,etc (10 or so) and maybe one or two kills. Maps with lots of barriers (Hotel, Mercy) can easily sustain 3-4 Berserkers
Do not spawn higher tier enemies at the end of the round unless absolutely necessary. (i.e. Spawning as Boomer with everyone else surrounding the last enemy) Spawn as a Drone or wait out the final kill (you cannot wait too long due to inactivity kick)
Look for COG planted grenades near turrets or otherwise. Shoot to trigger to eliminate threat and get the Sapper medal. For Heroes, down first if possible then wait for another COG to revive and then shoot grenade.
Spotting enemies (Left Trigger+Left thumbstick push-in button). Always spot enemies. If gives you assist points, and lets the team be aware of immediate dangers or enemies to focus on (i.e. Silverback, manned Turret, etc)
Do not rush too far ahead of your team in early waves to gauge their experience. Good teammates with kill any COG that run behind you, inexperienced teammates will only focus on what you are shooting at and will let that running COG flank you or another teammate.
Downed COG make the best bait to draw additional kills (and danger) toward you.
Tickers are usually ignored while destroying barriers, Wretches can jump over them by hitting "A". This requires practice.
Wretches are not as effective after wave 5 when the laser barriers start showing up.
Kill steals are worthless, and it takes a whole bunch of steals to equate to one down. Heroes are worth more as a steal, but still not as much as the down + spot. In short, sight, down and kill your own enemy.
Level 1 & 2 barriers aren't worth the time to destroy.
Sentries (any level) are good to destroy and especially necessary for Flame Sentry based maps.
Sentries/Decoys/Turrets are similar in value to a kill
To get through any spiked or barbed barrier when there is no choice, do NOT walk through directly, You will take a long time to get through. Walk to the point where the barrier is attached to, use that side as cover and turn-out of that cover. If this cover is protecting you from enemy fire, you can stay there without taking additional damage from the spike/barb barrier.
If you are stuck behind a laser barrier there is usually another route you can go. In situations like Old Town where the COG spawn uphill at the roadblock spawn and you are completely cut off, you can do one of two things: By yourself (usually in the alleyways) Get the attention of a COG by luring him (warning shot) past the barriers and down them so they have to crawl, opening all the laser barriers for you. You will need to be patient, and down or kill any additional COG coming to their aid. Option two is to regroup with other team members. This will cause the COG to swarm toward the group. Turrets must be focused on immediately.
I know alot of people are going to argue this but Tickers and Butchers are useless. Tickers are good on large maps and the first 2 waves or so, but with half decent player using a Drone/Retro there isn't much point to spawning as a critter unless its laser barriers and no one can get through to the last downed Hero.
The type of enemy spawned can determine how much enemy fire is drawn to you. So if you spawn as a Drone and everyone else spawns as a Ticker, odds are you're the one with the "Shoot me" sign.
Standing beside a player who is in cover or vice versa is generally not a good idea once Cole and the Silverback starts to spawn on the COG side.
Do not stand near the Beserker when the Silverback goes into Rocket mode.
Anya is deadly with the mortar shot. She can also float around in kneeling position when there is lag, so don't assume she's completely vulnerable.
Be wary of getting trapped between laser barriers. You're essentially a fish in a barrel.
You do not need to die and spend money to spawn to get to the next Beast Tier. I don't know why I keep hearing people say this in-game.
Your cash available or cash spent does not equate to your earnings. You also make earnings as a team.
Not reviving someone right beside you is a crappy move.
You should face away from the laser barrier while in cover in order to "jump" it. (laser barriers at spawn, Checkout map). In addition to this scenario, Boomers and Kantus should actually move closer to the laser barrier and tap the A button to straddle the barrier. For some reason the jump/straddle button option only become available at this position.
Single cover positions where lasers are directly on both sides can not be jumped (Gridlock) [for me at least]
COG, just like the Locust in Horde mode can do the crazy Z pattern running when they are charging you. It just seems like it happens more often. So if you are hard aiming (holding down Left Trigger) and they start to do this, switch to hip firing and use controlled bursts
Lag is more pronounced when using the Beserker. Your match may seems alright until you get the Beserker. Since the screen does not always update your position quick enough during a run, you must aim dead center between map barriers to avoid the phantom barrier (you see on the screen there is a path to run but you're actually running into a map barrier/wall).
Boomers cannot shoot downward with waist-high obstructions. Unless that reticule turns red, don't fire. You have clearance of around 35-40 degrees below horizon to fire.
You are never completely out of ammo. If you somehow run out of ammo, wait around 4 seconds and a small amount will be re-clipped. Those 4 seconds are long so make sure you get cover.
Retro Lancer takes awhile to get used to but is extremely effective. short 2-3 shot bursts, half second wait, repeat. You will instinctively want to hold it down during heavy enemy fire but take your time. if COG get near your cover, blind fire.
Spawn accordingly to the wave and COG spawn location. You'll find that even though you can afford some of the larger Locust, sometimes its better to use a Drone or Grenadier for maps or COG spawn locations where there is little cover for larger Locust. (i.e. Checkout)
Playing for Leaderboards:
Tickers are great for early waves, but as the Heroes start spawning, they can at most just get a down. They are not useful past Wave 6.
Drones/Retro are good all around but their effectiveness is reduced when separated by laser barriers and downed Heroes. Combination with the Grenadier and/or Boomer resolves this.
Kantus good for keeping everyone in the fight and protecting some of the Boomers
Bloodmounts are quick, and with the rider, can provide next target acquisition but are large targets to turrets being that they are front-line type offence
Boomers are great for taking out turrets, Silverbacks, and multple COG. Enemies can be taken out by the splash damage, so even if you have enemies behind cover, aim for the closest structure to them. Take your time getting the active reload or wait for the full reload. Missing reload or the finishing shot will cost alot of time and will allow COG to be revived and/or scatter.
Grenadier as mentioned before makes a good combination with any long range Locust, or launching offense with the grenade throw. Practice your soft aim/blind throw.
Serapede if good for causing a distraction for front-line offense or cleanup. Not so great on latter waves as the Heroes tend to barrel roll alot and unless you have backup, is time consuming.
Berserker..destroy all threats first (Silverback, turrets, sentries, Onyx Guards with One Shot or Flamer, Prescott and Hoffman), Take down barriers for your team's path and then COG. Only time when you should break this rule is if you have a guaranteed opportunity to take out several enemies at a time, otherwise its like the Serapede...you'll be chasing barrel rolling COG all over the place.
Armored Kantus, should be positioned between enemy and Boomers. Watch for COG attempting to flank the Boomer and stay close to cover to heal and revive.
Ideally there will be 2 or 3 Berserkers at the end. Even with just one, it provides enough distraction that teammates can simply walk up and clear enemies with little threat.
Focus should be getting the Onyx Guard Flamer and One Shot. The more they slow the Beserker down, the longer its going to take to finish
Playing for Earnings/Medals:
General rule: down and kill every enemy you can from cover, destroy all Silverbacks, sentries, turrets and decoys. Do not ignore the Silverback. It sucks to get killed by it twice. Do not bother trying to steal someone's kill. There's always something else you can get more points from. If the enemy is behind laser barriers and no one else can get to them, you go for it. Ultimately its a team game, but everyone wants to make the heavies so ya.
There are certain paths depending which Locust you spawn with and as mentioned before, sometimes you don't spawn with the most expensive Locust.
For example, I may have the Berserker on Wave 7 of Bullet Marsh, depending on the lag, and how much I am earning, I will stop using it by wave 10 (Flame Sentries) and switch back to a Boomer until wave 12 and by then I can sacrifice possibly two berserkers (One is almost guaranteed to die under lag).
Drones/Retros are good all around.
Wretches can be great when there are also multiple drones spawned (distraction)
Kantus are good for downing multiple enemies with ink and the points from healing/revive is also good if other players are using heal-able Locust
Bloodmount...kinda iffy on this one...the melee can be inaccurate under laggy circumstances and basically you're a bullet sponge
Mauler great overall, but since its a large front-line Locust, unless there is a Tier 3 or Tier 4 Beast on map you might be the one getting the short end of the stick.
Boomer...You can have this as early as Wave 5 or 6. Go for the Silverback first, crowds of Cog, Sentries, then decoys, then turrets. Always stand near a tall cover. Some COG heroes will target you first, you keep an eye out for them. They will literally roadie run toward you.
Grenadier good short range and long range capabilities
For Tier 4, hands down Beserker. I hardly bother with COG, just running a path of destruction is fun enough.
Typical COG/Weapon combinations: (I'm just recalling this from memory so it may not be 100% accurate)
Marcus - Lancer : Moderate
Dom - Shotgun : Dangerous at close range. (long range when lagging)
Dizzy - Shotgun : Same as Dom
Anya - Lancer/Mortar : Dangerous with Mortar in tight open areas
Cole - Boomshot : Dangerous to everyone WHOOOO!
Prescott - Gorgon/HOD : HOD is a threat to everyone but usually the Beserker
Hoffman - Boltok/HOD : Same as Prescott
Griffin - Sawed off/Boltok: More dangerous with the Boltok than the Sawed off
Sam - Retro : Dangerous during Retro charge
Jace - Lancer : Same as Marcus, but seems to chainsaw alot
Baird - Lancer : Same as Marcus
Orange clothed Older Male Stranded - Shotgun : Same as Dizzy
Orange clothed Male - Gorgon : Actually pretty dangerous in close range.
Orange clothed Female Stranded - Retro : Same as Sam (has hilarious taunting line)
Bluish clothed Male Stranded - Retro : Same as Sam
Bluish clothed Female Stranded - Boltok : Moderate threat from medium - long range. When two are shooting at you at the same time, its very dangerous.
My style of play:
I usually use the Drone/Retro from Wave 1-5, Boomer for 5-7, Berserker for the rest. The wave at which the Berserker is available is completely based on a few factors. How many downs, assists and worthwhile fortifcations destroyed. Kills do not mean anything. If you maintain 1st or 2nd earning in each wave, odds are you're going to have the Berserker early. An awesome team is when everyone is flat even and we all get Berserkers at wave 12 (This usually ends up with the good leaderboard time). I'll typically choose the Play to Earn style, but like I mentioned before, if the team blitzes the first wave without anyone dying, I'm in to try to make a better time.