Above the Influence - 100G
Adventure Mode - Finished a game where neither player drank Sleg.
Sleg is the silver liquid referred to in the game which makes you pretty much invincible. There are moments throughout where increasingly harder waves of enemies come out and there is a large vat with sleg in it to tempt the players into drinking it..
avoid drinking if you want the achievement. The battles may be tough to get through without it, but work together using battle charge and spell links on one another to get through these waves.
If you or your partner drinks it by mistake, reload checkpoint and you'll still be able to get the achievement. When you finish the game, at the start of credits the achievement will pop.
There is also a sister achievement to this one where one player has drank the sleg..
If you (or your partner) have drank the sleg and finished the game on your first play through, you can replay the levels on the current game save without drinking them. No need to create a new game or Adventure+ in order to obtain. Thanks to FailedSeppuku for that info.