for MW3 & Mercenaries:

my standard mech setup

Timber Wolf
6 to 8 ER M-Lasers (or clan equivalent)
2 ER LRM10 (or clan equivalent)
2 to 4 jumpjets
lots of heatsinks

main battle tactic:
long range - soften target with LRMs
medium range - hit & run using lasers in series mode (legs if possible)
short range - concentrated laser fire on legs while jetting sideways