How to unlock the 6 hidden achievements.
Other Door Kickers: Action Squad Guides:
Hidden Achievements Guide
Kill yourself with your own explosives.
Kill 50 Hostages.
Use 5 extra lives in the same mission.
Stay With Me:
Resurrect another player 5 times in the same mission.
Kill 50 rats (melee).
In the Nick of Time:
Defuse a bomb in the last 3 seconds.