To unlock this achievement you must "As Xbalanque get a fully stacked passive before the fifteen minute mark in a game".
This isn't an extremely difficult achievement to get. His passive is that after 1500 damage dealt (after protections) he receives 1 stack with a max of 6.
To sum it up, you have to get 9000 damage dealt in the first 15min of a game. I would recommend either running Xbalanque in Arena until you get it, or running the game mode of the day which has the possibility to give you damage increases or tons of gold at the beginning that will give you a better chance to unlock it. Good Luck!