Dont worry, i dont know if you found out the glitch yet but whats easiest is do the campaigne in coop up till the last chapted in each act on easy or normal and then when you get to the last level in each act, quit and load up again but put it on insane. Basically you can still get the achievement in for each act by just doing the final mission in each act.(http://guides.gamepressure.com/unrea...de.asp?ID=4299 - this link will tell you which mission is last and more).
As for the difficulty...Insane does exactly what it says on the tin, its damn hard. My tips include.
1) Just do coop with 1 other human, that way you can have more bots on your team (which are better than humans)
2)Complete the game once on easy to unlock the bonus cards, and then use them on a mission you get stuck on. One card reduces the enemy team by 2 bots, so they are seriusly helpful - i used that one on the first act last level.
3)Akasha will take at least 3 humans (good players, otherwise your teamates will just serve as cannon fodder and she'll fly up to 20 kills with ease)
4)One level is a DM on a spaceship, there is a teleporter that takes you to a distant turret, get in it and its an easy win - learn the level first in player matches or IA so you can get there quickly. Also have your teamate behind the turret with a link gun so he can fix it while your inside.
I dunno i'm sure you'll find a way, once your "in the zone" with the game its not that bad so longa s your teamates are good. I would offer to help you out but im kind of sick of the game now. That damn powerup achievements glitched or something and the harsh amount of time i've spent looking is criminal. I'm only playing now to get get my 200kill/day, and until the powerup thing is fixed i'm not gonna bother with ranked camp. Great game all round tho, certainly *****