Retaliate Version:

I used this guide to get my hopes up and general boss guide, but in all honesty, this fight is pretty easy. Esp if your retaliator and Vivi are 65. I only used sephs RW (but I put it on Cloud lol). Rest were synergy crap.

Cloud -- retaliate, thundara strike, Composure Materia

Vivi -- use his default SB; two thundagas, double strike materia

Wakka -- power breakdown (2 charges is enough), boost

Pally Cecil -- armor break (full), double strike

5th member -- anyone that can use double strike and at least cura. The attack value doesn't matter.

Friend Summon -- Sentinel.

Put everyone in back except for Cloud. Give cloud the best attack gear, don't worry about synergy defence. Just attack. He will never get hit except for the first few boss attacks.

If your thundara strike is Rank 5, use it on the mobs along side vivi.
The thundara strike will also one-shot mobs with lvl 65 cloud.

1st round, just alternate Vivi and Cloud with lightning abilities.
Don't Sentinel; just S/L if Cloud gets hit.

2nd Round, try to only use vivi's lightning. Keep at least 4 charges of Thundara strike for boss.

on 2/3 after one mob is dead, or big mob has been hit by Vivi once -- sentinel with whomever. This preps for boss.

BOSS

Defend with everyone, except for Vivi and Wakka. Remember that after you defend once, you don't need to defend the next turn, as the defend carries over until another action is performed.

Wakka, power breakdown, and then defend. Do it again if it feels it runs out.

Vivi -- focus first turn. Thundaga attack core until dead. Don't bother focusing again after it wears out. You can though.

5th slow heal guy -- heal vivi or cloud if they get low before attack core is dead. After attack core is dead, never heal regardless of how low they are at this point.

Everyone else -- get sentinel back up after the first one dies out, but defend otherwise.

After attack core is dead -- retaliate, boost, armor break. Make sure armor break is done before the first retaliate hit, because cloud might retaliate on the other core otherwise. If it is healed to full before retaliate can happen, just get in a Thundara Strike first.

Retaliate until dead. Double attack Materia on vivi makes him more useful.

Otherwise,

S/L if you think you took non-sentinel dmg. Remember how many turns it took before your Sentinel ran out. It will be the same next try as well.

S/L if your retaliator or anyone hits the defender drone. It's a pain to get retaliate to hit CPU after that happens.

Don't forget to click on CPU before you armor break. It still has a chance to hit the defender core at any point if you don't.

It may seem like you're taking a long ass time, and you are. However, you will loose 4 medals MAX. Still able to get Mastery at that rate.