Gift, Mangle. You want mange to have the most amount of uptime after Gumpling dies. Bring in Squirky after Grump dies.
If there is still cower left, bubble. Then stampede.
When Squirky is dead, Yeti. Rampage. Should kill him after 2-3.
Was able to do it with Terky (1-1-2), Kun-Lai Runt (1-1-1) and Harbinger of Flame (1-1-1). Start with jab, stampede, then repeat until terky dies. Swap to Kun-Lai Runt and Rampage and thrash until he dies (he seemed to block everything after rampage. With Harbinger of Flame, pass until his crouch is over and then use Impale, which should kill him.