NOTE: This method can also be used to earn an S Rank for a 2-for-1 deal.
For this trophy you'll have to earned an A Rank for The Last Battle between Kyuubi Naruto and Killer Bee against 6 Jinchuriki (Tailed Beasts) and Tobi.
For FACTORS on reaching high Rank for Boss/Pseudo battles please consult with this trophy solution:
This battle is EXTREMELY difficult to S/A Rank if the right precautions and methods aren't taken, as it is a 4-part battle with individual ranking, for an overall rank. Primarily you'll want to bring plenty of healing items! By this point, depending on how far you've leveled your Hero palette, you should be able to use All-Purpose Ointment or Wonder Elixir. HEALTH is a MAJOR, if not only, factor is obtaining an S Rank across ALL of these battles, so be aware of it and heal only when necessary.
For the first part of the battle you'll have the Ultimate Decision of battling the 6 Jinchuriki at once, or split in 2 groups of 3 by Killer Bee, for the sake of the S rank choose Hero. Here you'll battle the 3, 4, and 6 Tails, in human form. You'll want to get rid of either Roushi, for his magma pillar can get annoying, or Utakata, for his bubble spamming. Mix up the lock-on to sub away from the other enemies attacks as well. Your Planetary Rasengan Jutst will do wonders as well for whoever is in the crossfire of it's path. Before you've dealt the final blow to your last enemy make sure to heal ALL of your heal, as the next battle you won't be able to, at all. You'll need green health (60%) to be granted an S rank for this individual battle.
This second battle is, and for most, the most difficult to overcome for a suitable rank to get an overall S, as I've had to retry this battle over 7 times for my D ranks. This part of the battle you'll be playing as Naruto with the Eight-Tails as your support, against the 6 Jinchuriki's Tailed Beast forms (2-7 tails), fighting 2 at a time. For this method you'll have to give into guerrilla warfare of attacking and running around. Also when a Tailed Beast is charging their Chakra for a special attack, delaying them with a few Windmill shuriken and use Killer Bee to interrupt and cancel the attack.
The first part of this Tailed Beast battle will be against the Two and Three Tails. You're going to want to take down the Three-Tails first, as he has projectile attacks and an annoying Roll-Out attacks that almost unavoidable. While maneuvering around the basic Two-Tails and Three Tails' attacks, hail down Windmill shuriken in the air (Jump+Y+X), doing so a few times in rapid succession will cause him to pause while being damaged. Be sure to also use Killer Bee when necessary. Also playing Cat-N-Mouse (No pun intended) with the Two-Tails charge your chakra until you have enough to go for more Windmill attacks. Keep doing this until you've dealt with the Three-Tails, then proceed to do so with the Two-Tails. Once the 2 Tailed Beasts have retreated you'll partake in an IA sequence and a cutscene will the ensue. Once the cutscene finishes you'll engage in a brief fight with Tobi.
The second part of the Tailed Beast battle will be against the Five and Six Tails, this part will be a cakewalk. For this battle you'll want to hail Windmill shuriken on the Five-Tails (Horse) because the Six-Tails is extremely slow and stationary and it's bubble attacks won't reach you if you're on the opposite side of the battlefield. Just be careful of the Five-Tails' Charge and Smash attacks. But just perform the Windmill method, using Killer Bee when necessary until the Five Tails retreats, then go in for the kill with the Six tails. After this you'll partake in another brief battle with Tobi.
The final part of this Tailed Beast battle will be against the Four and Seven Tails. This last part you'll be dealing with the Seven Tails first as it remains stationary, but throws these ANNOYING air blasts that can reach any part of the battlefield. However your Windmill shurikens can break the air blasts and still go in for damage. You'll just need to out-maneuver and dodge the Four-Tails' Molten Rock and green aura blasts. Once the Seven-Tails has retreated just wrap up business with the Four Tails, Windmills and Killer Bee. The last part of this battle will be a brief fight with Tobi, again.
The third part of the overall Boss Battle will begin with a cut-scene to which you'll gain new power and trust with the Nine-Tails, or Kurama. You'll then transform into Bijuu Naruto and take control of the Nine-Tailed Fox for this fight. Here you'll be going up against each Tailed Beast individually. You'll want to use your Tailed Beast Bomb (Y+B), this cuts their health in half, so 2 attacks will do. However the chakra charge is long. if they do get too close just resort to melee attacks, otherwise just use Tailed Beast Bomb twice per. Then you'll need to approach each one and initiate a finish move with "B". This battle will finish once the Seven-Tails has been defeated.
The FINAL part of this ENTIRE Boss Battle will be Naruto against Tobi. You'll be able to heal yourself in this battle as well. The battle will be pretty standard, in Mob battle form. This will go on until you trigger an IA sequence, follow the on-screen prompt and deliver the most righteous, ultimate, mega, super, every-person-in-the-world, Father-Son, Super Dragon Fist, PUNCH OF DOOM! This will finish up the battle and based on the previous battle ranks, and if you had enough health you'll receive an A Rank!
If, for any reason, you aren't able to earn an A Rank no worry. You can retry ANY battles post-story in the Ninja World timeline for another try.