Solution taken from Clement_RKG on Steam:
https://steamcommunity.com/sharedfiles/filedetails/?id=32164...As the title suggests, you will have to survive for 20 minutes in endless mode. For every timed achievement like this one, they recommend enabling the 'in-game timer' in the gameplay section of the options. Their advice for this achievement is to use a character you are comfortable with, opt for the Multishot, and always keep a 'heal 20' available in the shop. Use it when your health is low.