You need to survive a wave on 1% hp. I find it easiest to use the beam upgrade so that you can fire instantly and keep it pointed at a single basic ranged enemy while you watch your hp bar go down. Wait until you only have the tiniest sliver left, then kill it. If it doesn't work, wait until you get another one in another wave and let it hit you one more time. Repeat until you get it. Normal difficulty and the shield are helpful as you can ensure you're only taking damage when you want to.