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I was struggling to find a spot to do this, but here's what worked for me:

1. Turn on the "Riot" modified (+50% Enemy Modifier)
2. *Optional* Have a high-rate of fire gun and the "Cold Feelings" Cryo Modifier (Shooting inflicts CHILL around you w/ chance to freeze)
3. Get to the third biome (snow covered)
4. Hope for an outdoor level w/ a breakable wall and a few shield-guys
5. Lure the shield guys, and whatever other mobs spawn that will follow you to the breakable wall - shoot away from them and freeze them in place while you wait for more to arrive. **Try not to shoot the breakable wall until you're ready otherwise the shield guys are liable to yeet over the edge by themselves**
6. Knock off 5 when they pile up.

Took me a few tries to get the right mobs on the right map, but it'll work eventually