Starters: Grommosh (2), Valeera (3), Varden (3)
Bench: Jaina (3), Cairne (3), Diablo (3)

Build from Dallas, who won the Last Mercenaries Standing tournament with it. see here: https://docs.google.com/spreadsheets/d/1TC9UE9kO9y0T8rYVdrtfE2xrftkUUtU4eY_4gvWEq5c/edit#gid=0

I used it on ladder, climbing from 10k to 11.7k. Sometimes you let Valeera sit in stealth doing nothing while your other mercs whittle down the opponent.