Here are some general tips for getting the unstoppable achievements full credit goes to Tracker4502
A) Doing it alone is always easier. You may think that having a buddy helps, bu the level becomes sufficiently more deadly in Co-op vs single player. However, the level is only easier if you are playing through the campaign; revisiting the level makes it the same difficulty as doing it in Co-op. Lastly, you don't need to make a new character to try campaign again, but just be warned that you only have once chance per level per campaign to try it on the easier difficulty.
B) Retreat is always an option. If you can retreat, do so. While you're at it, use pistols / rifles / cannons to pick enemies off one by one. I suggest lasers since they slow down your enemies. Retreating also allows you to break the mob up into small, more managable groups.
C) Never cut corners. By this, I mean don't waste your spiders, battle cys, ruiners, or sentinent weapons just to speed something up. Save all of these for emergencies. By keeping them all, you will be able to stave off the harder groups of enemies and never be left overwhelmed and cornered.
D) Patience is key. Between every mob, let your battle cry and spider meters recharge if you used them. If you are a bio-engineer, allow your health to refill.
E) Use your teammates as bait. The game will constantly supply you with a bevy of teammates that seem to be innumerable. Take advanatage of this by always standing behind them. They do a good job of slowing down the enemies and function as perfect meat-shields.
F) For Ice Forest and Helheim: Save a sentient weapon for the boss. In the final phase of each, a sentinent weapon will make short work of the boss. This means less risk of you dying at the very end.