This is an easy achievement, just don't drain the water in 'Cargo Hold' or 'Engine Room', survive the 20 rounds and achievement will unlock!
I'd suggest doing this on casual as you have 250 health and zombies are slow, also try and get the Kraken from the random box and pack a punch it (even with a Catalytic Detonation) as you can comfortably survive 40+ rounds with it. With this set up you can literally camp or move anywhere you like, just keep shooting the kraken when there is a bunch of zombies together and/or you've had 2 or more hits. You should end up getting 'Kraken Unfettered' and even 'Rock, Paper, Plasma' achievement while doing this set up, so it's a highly recommended set up if you have none of those achievements!
Hope this guide is helpful! :D
*NOTE* Achievements can't be unlocked if using modifiers, when going for achievements use classic!