Undead Brawler

I actually went 10-0 with the following team:

Blightbreath: Breath - Gas - Rain
Scourged Whelpling: Sweep - Darkness - Breath
Widget the Departed: Pounce - Spectral - Darkness

Basically whenever the other team can't change the weather or you delay your own until the opponent uses theirs, it feels like you can't lose.

Always open with Blightbreath and use Rain => Breath => Rain (can open with Gas spam if the first pet from opponent has a weather ability and wait for it)
After Blightbreath dies, bring out the whelping and spam breath as long as rain is active. If you happen to face magic pets you can revert to Tail Sweeps or use Darkness if the other team has too much healing.
After the whelpling dies, widget should have no problem cleaning up the rest of the enemy team after they've been slaughtered by AoE. They will either all be dead or super low on HP, and widget is one of the fastest undead pets. He should easily be able to kill everything with Pounce spam or a Darkness + Spectral combo if they're still at 400-500 hp.

I'm using an alternate strat from mm0p's, considering I don't have 2 Blightbreaths.

Unborn Val'kyr (preferably H/H) 2/2/2
Blightbreath 1/2/1
Scourged Whelpling 2/1/2

The whole goal is to get off Curse of Doom on your target and spam Shadow Shock until you hit your immortal phase, then Unholy Ascension the entire team. If you can get this off, your remaining 2 pets will WRECK the opposing team. I have had moments when the backline pets have died before the pet you're currently fighting.

Completed this with 100% win ratio using:

[npc=115148] 1/1/1
[npc=115148] 1/1/1
[npc=62854] 2/1/2

Prio: Acid Rain > Dreadful Breath

Most AoE undead pets will work here. I would advise you do forfeit if you go up against any 100% beast teams.