I defeated the Cursed Effigy 6 times now but I can't seem to trigger the achievement. Do I have to go somewhere specific, or is it a bug and I should file a ticket?

Seems like a lot of battle achieves are working wonky after battle reset, I had the same problem. Just file a ticket and they will add the achieve for you.

Thank you; maybe I need to check if I "missed" other battle achivements then D:

EDIT: Hmm, the normal challengers give achievements just fine... I'll write that ticket now ^^

It seems like the ones that are broken are just the ones that existed before this recent reset. Meatslab's achievement is also broken, for instance