Rollerdrome

Probably not one you'll get on your first attempt but not too difficult if you replay a level a couple times. I recommend either the first level of act 2, or one of the mall levels.

Make sure you memorize and utilize the Combo tokens as they buy you time, and leverage Perfect Dodges to enter Super Reflex as it boosts your damage and makes enemies like the shield guys vulnerable to your standard pistols. Don't be afraid to use your grenade launcher for easy kills to keep the combo going, and save the explosive barrels for the shield boys as you can spam rt and kill them in 3 or 4 pistol shots instead of your whole mag.

If you're struggling, play the game naturally until you unlock the charge rifle/railgun thing. That will get you out of sticky situations by letting you get emergency kills in Reflex mode if your combo is about to expire.