Put Spirit Fox in your team and zap Infernus’ mana.
Use Mab or any other Freeze troop and stop its extra turn.
If Infernus is in first or second place, use Emperor Khorvash to drain and stun it.
Or put Great Maw in first place, Mercy in last place and, with any luck, you’ll get a “Yellow Special” so when you convert Purple to Yellow you get a 4 or 5 Match. Add in the Holy Banner for Yellow ++ and Maw gets filled pretty quick and then Devours Infernus (or Ubastet if it’s around)