I'll bump this because I got a question about mercs. But I'll also answer this one since it appearantly hasn't.
Urban Chaos: Kill 15 enemies with counters = Red dude with shell (the one you have to fight about 10 times on steel beast)
Steel beast: Get 25 coup de grace kills = bee hyve queen thing
Mining the depth: 70 kills in 10 minutes (for both single and multiplayer). Does not have to be a combo. This is the easiest one I suppose. = That lizard dude that spits.
The other maps I don't know.
Now, my question: I noticed, when you do multiplayer mercs, you can both take a different skill. Do they stack or not? So if you both take the time bonus one, do you get 20 additional secs, or just 10? And if it doesn't stack, does it only work for the one that took it? Like, when one takes time bonusses skill, would it be best if he got all the time bonusses?