This achievement is fairly easy but a bit time consuming as the Necromancer has only 50 hp.
You will want to load up the map "The Vacant Sea" on Hard (You will die in 2 shots instead of 3)
Once the map loads. Just walk into the spinning blade until you die, respawn and do it again. Once you are dead and the option to retry pops up, select it and repeat. You will need to do this 100 times as you only lose 50 HP per death (100 before failing and retrying)