This is a story achievement so you should pick it up on your first playthrough. The game glitched on my as I approached the big gold door on first run through though and I ended up going under the map and died then restarted right after the boss fight. I've not read too many reports of the happening but it worked fine on my 2nd playthrough.
The boss fight itself is pretty easy. Just beware that the outer platform rings will drop away every making the fighting area smaller. Just stay towards the centre and this fight should go easily enough and you'll pop your 25G.
Check out the video below if your still having issues.
I didn't record this so kudos to the original uploader. The fight begins about 50 seconds into the clip.