Load up the Broken Citadel level on default difficulty. Near the start you should be able to find a Void Stack. Clear out any mobs around the area and then stand in the Void Stack. You will begin to gain the voided effect, which will slowly stack to 999. You will need to heal through the damage as you wait for the full stack. Once you reach voided x999, step out of the stack and wait for the effect to end (90 seconds). As long as you don't die during this process, you will get your trophy