The Can't Feel a Thing challenge in Caustic Caverns requires you to stand in the acid pools for 30 seconds without entering Fight for your Life mode. When I first got a shield immune to Corrosion damage I thought I cracked the secret. Surprisingly, corrosive-immune shields don't actually protect you from the acid. And at level 38 the acid is still quite painful even in Playthrough 1.
How do I get this challenge? Do I need max level and a crazy shield or is there some trick to it? FWIW I'm a Siren and could conceivably boost up a Turtle Shield with skills and a Class Mod but ideally I'd like to know a way all characters can do this.