"Defeat" is a little inaccurate, as this one is more about surviving the encounter for long enough but here we go...
This battle is broken into three sections. In the first, you begin alone. Basically, run like fun. You can't damage these guys no matter how hard you try, but I was firing at them so you may need to do this anyway. Stay away from the danger zones (hit B to dash if needed) and keep on the move. Eventually, play will pause with a cut scene and Gamora will join you.
At this point you can start dealing out damage, but again it's all about avoiding your own death. Try to separate the two Brothers - you'll see their armour disappear - and you can wail on them. Use Gamora to attack one brother while you attack the other, or use your ice attack to help separate them. Melee attacks on an armourless, frozen Brother does decent damage. Again after a while, another cut scene where Drax joins in.
Continue as in the section above, only adding Drax into the mix with his attacks. I managed to get both Brothers down to around half health when the battle was ended and the achievement popped as... cut scene!