put 4 pokemon in each chamber such that one type from each pokemon lands a super-effective hit on the pokemon clockwise of it. example:
rock/ground > normal/flying > grass/poison > water
rock is SE on flying, flying is SE on grass, grass is SE on water, water is SE on rock. set each chamber up like that and you should have no problem.
if that's too complicated, just catch 36 hoppips at the oasis pokespot and put them in all the spaces in the purification chambers.