Also possible in the chapter where you need to be stealthy, and not let the guys blow the horns. Let em blow the horns, it will give you more guys to try it on. The mortar is before you go thru the gate.
Its tough"er" by yourself. Just have a friend lancer down the guy as you're walking over to them. Do it on casual and it wont be as easy for you to go down. All the heavy weapon executions are pretty boring tho, all the same...
Second Chapter in first Act when I was Cole and running on the bridge. After I ran to the end of the bridge where the Locust mortaring me, I killed all but one. I downed the last guy, picked up the mortar and executed him.
i just had a friend load up 2-3, where they blow the horns. in that tiny little area, i was able to get the executions for the digger, one-shot, mortar and the cleaver. if you down an enemy, you have to go pick up the heavy weapons and walk over to them.
the AI almost always executes your down before you have time. so as it was said here, put it on casual and walk out near the locusts and have a friend lacner or hammerburst them down and be ready to hit Y. it's hard for you to go down on casual and also your
partner is shooting them / distracting them so that's the way to do it.