Last missing achievement

[Tartempion]

Do you have indices for the last achievement of the game (hidden, currently at 2.4 global completion rate)? I don't want to reveal the spoiler on Steam if it is a boss fight (although I doubt it, I reached true ending)...

[Superlucas1231]

Are you asking if the last achievement is a spoiler? Because it really isn't.

It's What a Technique. It requires that you kill 10 bosses with artful gambit artifact on. Given how you beat the true boss, it's likely you have to go onto Newgame+ to get it.
The artifact itself can be found near a boss in the Cerulean Expedition, and other artifacts can be still equipped when attempting the challenge

[Tartempion]

Yes I didn't want to look myself in case it was a story or boss spoiler. But it's not, thanks for your answer.

[Zalugar]

I don't know why this particular achievement is hidden to begin with. There's nothing particularly spoilery about it.