If you're having trouble, go with Heroic Difficulty and the following Skulls:
Take Tough Luck too, if your tactics are suited for it.
If you can hold your ground on Heroic difficulty, you'll be fine with this. You won't come out with two many points, sure, but you also won't die very much as this is a fairly harmless combination of Skulls.
I would normally go Legendary with my six favourite Skulls, but if you're after partners to help this isn't really a good idea. Still, with the above suggestion you should be good.
Remember also, of course, that Headshots, Sticks (most of the time), beatdowns, Assassinations, Needler explosions, splatters and the Power Drain/Plasma Pistol all get you additional points, as does a rapid multikill of three or four enemies.
I have a lot of experience helping people with the Campaign-Meta achievements and I'd be glad to help, but it is of course easier solo.
There are a few basic principles to getting the achievements, first is getting the time bonus at the end of the level which multiplies your total score by 1x,1.5x,2x,2.5x or 3x. You earn medals for special kills such as
EMP, Assassination, Headshot,
Splatter, Needler Supercombine, Grenade Stick and Multikill
which involves killing 3 people in a second. which grant
1.25x the original amount of points awarded for the kill.
For solo I would recommend Normal difficulty with Tough Luck, Catch, Fog and Thunderstorm as it gets your more than enough points.
On co-op I would recommend Heroic difficulty with Catch, Fog and Thunderstorm.
I'm down for helping, I'll add you and send you a message when we're both on.
Yes, at the end of the day you have to be at very least a competent player who is capable of getting a few head-shots and sticks. I tried to get it with a player new to the game and it simply didn't work because he couldn't get the points.