I just got this x3 and I know some people are struggling with it so I figured I'd post my team.
Feel free to post the team you used to get this achievement C:
Lvl 21 Maggot - Beast - Consume, Acidic Goo, Leap
Lvl 21 Fel Flame - Elemental - Burn, Immolate, Conflagrate
Lvl 22 Flayer Youngling - Humanoid - Blitz, Reflection, Rampage
I basically opened with Fel Flame and nuked the first pet. Then I'd swap out and use one of the other two depending on what I come up against. Maggot is a beast with a damage increasing ability, so when its under 50% its heal can crit for almost 500hp which is amazing for a 1 round cool down ability. Flayer Youngling's Reflection is amazing too, especially if you time it to reflect a heal or big damage ability back on the other player (speaking of which, do any other pets have Reflection?)