This solution is for the SUPERSTAR achievement and it will be 100% spoiler free. I'm not going to give the name of the final boss. However, when the final battle starts, you fight one boss. After a cutscene, you'll fight a different boss twice. It's the second form of the second boss where the 5 star rating is needed. I got a 4 star rating on the first form of this boss and 5 star on the second.
I just wanna add another strat. The other 3 solutions are top-notch and I thumbed them up before posting, so be sure to read their solutions if mine doesn't work. There are many ways to go about getting this achievement, so if one of our solutions do not work, try another. If you find a different way, then come post your own solution. The more information we get out to people, the better chance they'll have the achievement and that's the point of TA's solution section.
Anyway, I wanted to get this achievement before completing the game, so I reset the fight many times to try different strats out with different party members. My weapons and accessories weren't the best, but they weren't bad either. My final party settled on Light, Fang and Sazh. I'd post a video of this fight, but just watching a video won't do much for ya.
You should be fairly familiar with upgrading. If you need some help, check my starter guide post out here:Upgrading questions in FF13 (Upgrade starter guide inside)
There is undoubtedly much better guides somewhere on the internet. I made this starter guide after only on night of toying with it and it's only meant to get people started, not craft the ultimate weapon. So if you're looking for *the* guide, you won't find it in that thread hehe.
++WEAPONS and ACCESSORIES++
-Flamberge (tier 2 starter weapon)
-Power Glove (tier 2 Warrior Wristband)
-Power Wristband (not upgraded)
-Rune Bracelet (first * level)
-Glaive (tier 2 starter weapon)
-Warriors Wristband (not upgraded)
-Power Wristband (not upgraded)
-Glass Orb (not upgraded and useless)
-Procyons (no upgrades)
-Sorcerer's Mark x2 (no upgrades)
-Weirding Glyph (no upgrades)
I'm going to list each character's strength and magic stats to give you a feel of how your character compare to mine. If you're going to use my solution, then try to get them somewhat close. However, Lightning and Fang's strength stat and Sazh's magic stats are the only stats that matter. If you went the Commando route with Sazh, then strength will matter more than magic. Any extra points you have in the main stats (above me) will only help you even more.
-Lightning: 1643 Strength, 1258 Magic
-Fang: 1831 Strength, 794 Magic
-Sazh: 573 Strength, 945 Magic
You only need 3 paradigms:
Character order: Lightning/Fang/Sazh
1. RAV/COM/SYN (Decimation)
2. RAV/COM/RAV (Relentless Attack)
3. COM/COM/RAV (Aggression)
3a. COM/COM/COM (Cerberus) if Sazh has higher strength than magic
The target time for me was 2 mins 51 seconds and my completion time was 1 minute 56 seconds. Make sure you have at least 4TP before entering this battle because RENEW from the TECHNIQUES menu is how you'll heal your characters. 4TP allows you to cast RENEW twice.
You cannot damage the boss until it has been staggered. So don't be alarmed when you notice there is no damage. You need to kill the boss during the FIRST STAGGER (providing you staggered it fairly quickly) if you want the Superstar achievement. If you don't kill it during the first stagger, press start and then back to retry. Get a stopwatch or use your phone/clock/watch to give yourself an estimate on how long it's taking you to win the battle. I started my timer the moment I saw the boss during the cutscene which ended up being about 10 seconds before the timer starts (I think it starts when your character has DOOM cast on it).
Start off with Decimation paradigm (RAV/COM/SYN). Stay with this paradigm until everyone has HASTE and Light and Fang have BRAVERY.
NOTE: The boss has an ability to SLOW your characters. This will remove the the HASTE buff (or cast SLOW on your characters without the HASTE buff). If you lose HASTE, then wait for Sazh to cast HASTE again. If you're in another paradigm, swap back to RAV/COM/SYN until HASTE is cast on all your characters.
Once those buffs are applied by Sazh, swap to Relentless Attack paradigm (RAV/COM/RAV). What this does is allow you to build the chain gauge faster.
When you stagger the boss, switch to Aggression (COM/COM/RAV). The boss will launch into the air and you can air juggle the boss, but it CAN STILL ATTACK. So keep an eye on your health. If you don't kill it during the first stagger, just retry and go for it again.
The brief solution:
1. Make sure you have 4TP so you can use RENEW to heal your party twice if needed.
2. Start with RAV/COM/SYN until everyone has HASTE and Light and Fang have both HASTE and BRAVERY.
3. Swap paradigms to RAV/COM/RAV until the boss is staggered.
4. Once the boss is staggered, swap to COM/COM/RAV to kill the boss before the first stagger is over.
5. *Achievement Chime*
Good luck everyone, hope this helps and, as usual, sorry for the long solution.