Worked AWESOME for me:
Tranquil Mechanical Yeti
Key skills: Call Lightning
Clockwork Mechanical Gnome
Key skills: Build Turret, Metal Fist
Key skills: Corrosion, Acidic Goo
Turn 1, send out Yeti, use Call Lightning (all mechanical attacks do more damage, all attacks add an extra lightning strike). Oon will blast Yeti, sending it to the back row.
Turn 2-5, use Gnome to set up two turrets and use Metal Fist in between. The turrets deal damage three times each round each, so that's three lightning strikes per round. Each turret set up gets a total of 12 bonus lightning strikes. Gnome will likely be killed for good after turn 5.
Turn 6: Jade Oozeling sent out, use Corrosion to increase the damage of all attacks against Lil' Oon. The two turrets up will then hit a total of six times, followed by six lightning strikes, all amplified by the Corrosion. If Lil' Oon is still up, use Acidic Goo to do even more damage.