I am at Orphan right now and having a very tough time. I tried the strategy where you use only Medics, Saboteurs and Synergists and I still keep dying regardless of being in Med/Med/Med. Anyway, that's not what I'm asking... If I don't get the Superstar achievement (Kill final boss with 5-star rating) now, will I be able to fight him later or is this the ONLY time I can get that achievement?

The 3rd phase? And yes you will be allowed to return easily once you have beaten him.
 
Edit: Just to expand a little, I think you only need to beat the 3rd phase with 5 stars for the achievement, which I found pretty easy even after struggling with the first 2. Also once complete the game will expand your crystarium to max and will spit you back out onto pulse with a portal that leads right back to the final boss. So if you don't manage to 5 star it first time around you can level up a bit and give it another shot.

I think its designed that you have to beat him at least once to get that achievement.

Getting that 5 star achivement on the first try is impossible(as far as i know) you need the gold watch from beating the final pulse mission in the game. The gold watch when equipped ups the time needed for 5 stars and thus hands you the achivement :)

" The 3rd phase? And yes you will be allowed to return easily once you have beaten him.  Edit: Just to expand a little, I think you only need to beat the 3rd phase with 5 stars for the achievement, which I found pretty easy even after struggling with the first 2. Also once complete the game will expand your crystarium to max and will spit you back out onto pulse with a portal that leads right back to the final boss. So if you don't manage to 5 star it first time around you can level up a bit and give it another shot. "

" Getting that 5 star achivement on the first try is impossible(as far as i know) you need the gold watch from beating the final pulse mission in the game. The gold watch when equipped ups the time needed for 5 stars and thus hands you the achivement :) "