Try to engage them with something along the lines of a group of hive scum using autoguns. Tactics such as these can prove much more deadly than one powerful enemy and provide more balanced loot.
I almost killed my group on multiple occasions with a groups of 5 - 15 cultists/hive gangers/etc. with autoguns. Give them cover, let some of them rain suppressing fire and the rest uses overwatch or just snipes. Maybe some of them try to flank the players.
This made getting good loot from the big baddies all the more rewarding.