Au is an Elemental. I have 3 reasons to pick Spawn against him. 1. He's faster. 2. His AOE is VERY strong against mechanical, and 3. his Dive negates. Au's Gold Rush damage. So, start with Thunderbolt, this gets it cooling down and does damage to Lil B. He will attack with Gilded Fist. Your next attack is Dive, this negates the incoming Gold Rush damage completely. When you resurface, just spam Swallow You Whole until he's defeated. It has such a high hit chance you should easily take him out.
Banks is a Critter, so ride the Spawn’s damage out and use Thunderbolt again, taking Lil' B down further. Once Spawn is down, bring in the Kun-lai Runt and open with Mangle. Banks is going to try and out heal you so every little bit of damage counts here. Pop Rampage, and bring Banks low. Just spam Thrash till he's dead. Bacon Anyone?
When Lil' B enters the fray, he's at about 1/2 life - which is great. Continue to ride the Runt’s damage by using whatever isn't on CD. When Fel Flame enters the scene, it’s time for the classic Immolate-Conflagrate combo. Follow it up with Burn and Lil' B GETS OWNED!