i have 0 experience with mercs but i do have a ninja/pain at lvl 50 at that was an "interesting" experience and dont get me wrong you can make it work for you, but given the opportunity id go with thermal instead, pain domination can heal the hell out of your ninjas but it does get chaotic at times, thermal can give them slightly more durability, i tend to find that as they are mainly melee they will run off round corners to chase down and if u cant heal them...i know you said your only taking to level 32 but theres always that temptation to take it further...if anything for ninjas you could always wait until time manipulation becomes available it has an alright heal and the buffs would go well with ninjas....
