My understanding is that powercrystals is fixing it such that mobs like slimes simply won't spawn extra mobs if killed by the grinder, but he's fine with inventive thinking to get around this. For example, spawning a large slime and killing it via a vanilla fall trap where the mediums are then killed by another fall trap and finally the smalls by grinders would be acceptable. As would moving a regular spawner in and standing there, or using a soul shard spawner in conjunction with the auto spawner.