If you're entering the island first time you can do it by waiting for the big blocky mobs to shoot an orb and using the W skill to hit the orb they shoot back, a bit tricky to do but possible. If you already completed the story the only way I know possible is to do the witch instance and wait for it to spawn the monsters and try kiting them until the big blocky mob shoots the orb. This will be more difficult and expect for it to take long to complete.