I've seen that this is not working for steam and one other mentioning it wasn't working for the xbox360 but have yet to see anyone comment on the ps3. So I figured I would.

 

Just attempted this, took all the gear off both my main character (anjali) and the AI (lucas) while I was in the graveyard where you fight the witch. Killed her with default gear on both characters but no luck... Trophy didn't get activated. I'll try again with the next boss and see if I have any luck with that one.

 

This was on normal difficulty incase that information helps at all.

Ok, I fought about 3 more bosses pretty soon after that witch. I kept my gear off and on default all the way through to make sure that no attacks were made before I could remove the gear. Still no luck in getting the trophy, even after killing 4 bosses. So yeah safe to say it's broke.

Yeah, the devs seem to be working on it, at least for PC. Achievements should be rewarded retroactively, though, once it's fixed.

I got the trophy just now from doing it by killing Rajani. Perhaps there's a certain difficulty level for the bosses that has to be met for it. I can say for sure that that Rajani battle was far more difficult than any of the other bosses or sub bosses so far (any with a health meter).

Oh, perhaps Vera and others count as mini-bosses, and Rajani is the first 'proper' boss. In that case, though, it might only apply to a few battles - Rajani,

the First People chieftain, the Warbeast, Dapper Old Gent and the final boss.

If that's the case the achievement may need the wording changing a little to reflect that. It makes out that you just need to defeat a difficult opponent (no mention of it being a boss) which makes you think; any with a health meter.

 

Will have to get others to test it out on other major bosses to see if it gets results too.