Is this achievement winnable? Is it possible to kill the sea serpent and the tentacle monster in direct combat or do those not count as bosses?

The sea serpent is set as neutral now so u don't have to kill it for that achievement - u can even free him and let it go. Not sure if killing the tentacle monster is required for Bossa Nova but even if it is then no direct combat is needed here.

Bosses: Proach, Gog King, Pirate Captain, King Wraith, Drat Queen. If you lose combat to any of these bosses, it's game over.

I've killed all of those and I did not get the achievement. What about Uncle Shamus? Is he a boss?

I'm pretty sure it is not game over if you lose to the Gog King. I've lost to him and his minions before and I'm pretty sure Katie just finds you and takes you to Lady Moira.

I'm surprised the minotaur isn't a boss. It is game over if you lose to him in combat (and a pretty funny game over too. The minotaur uses Shawn's head as a wall plaque)

Hey, yup Uncle Shamus is a boss!,
they took out the Minotaur and the sea serpent from the achievement in one of the patches,
if you kill all of those listed above and Uncle Shamus that should trigger it! :)

Yeah, when I killed Uncle Shamus, that's when I got it. It is a shame to know that I killed the Minotaur for nothing though.

To clarify, Bossa Nova was *not* achievable until Patch 2 - we checked the wrong Wraith ID, so killing the Wraith wasn't counted. It works fine now, and you do *not* need to kill the sea serpent or the minotaur to achieve Bossa Nova.