Tifa with Premium Heart and a Shinra Alpha or Zeidrich makes things a LOT easier. 4x cut with like 6 or 7 Counters, Restore + All, and a Bahamut ZERO (or KOTR) + Final Attack which plays well with her Grit ability. Then throw on an MP Plus so that her MP is maxed out for maximum damage. Have someone cast Dragon Force on her and she'll tear through most things like paper. Thanks to the Auto-Crits from Dragon Force, her basic attack from 4x Cut does more damage than all 7 parts of her Limit Break sometimes. And run if you hit her cause she'll come back with at least 3 or 4 Counters and follow it up with another 4x Cut combo. Even without Dragon Force she is definitely a force to be reckoned with. Only struggle is keeping her alive, but that's where the other two come in.
Super Tank Barret with either a Warrior Bangle to further bolster his stupid high defenses, or a Fourth Bracelet to reinforce his low magic defenses and a Ribbon. He's the one that'll stay alive, and without any negative status's, or any for that matter, but his defenses more than make up for that. Give him the Masamune so he can keep up with Powerhouse Tifa, then throw on Heal + All to make sure the rest of the team gets all fixed up if you are hit with some nasty status's. You could throw on a Restore + All, but I feel Enemy Skill is a much better replacement. White Wind will keep everyone generally at max health thanks to Barret's super high health. Then, throw on a Revive + All cause he'll normally be the last one standing after a group attack (unless it's a spell, then you better hope you have Wall up, and the other two healed to tank it and then bring Barret back lol). Add on two Covers so that he'll take every single target phyical attack, and then fill the rest of the empty slots with counters so he can hit them right back after the Cover and you got yourself a damn fine tank! Careful of his MP though, those Enemy Skills drain his low MP super fast so have some Ethers/Turbo Ethers at the ready.
Now, this next one is per choice if possible. I prefer to have my 3rd character be Aerith. Her high magic, and Final Limit break are a fantastic edition to the roster. Give her her Princess Guard for those 4 double slots, and a Zeidrich or Shinra Alpha for even more fun 4 double slot fun. Princess Guards ability to give shield seems amazing at first, buuuut, in most fight's it ends up being a negative since most boss's can attack through shield. You are better off having her use Cure to heal. Master Magic + MP Turbo with a Mega All thrown in somewhere is the greatest thing for her. Throw on Restore + Sneak Attack and Barrier + Sneak Attack for quick full buffs at the start of the battle so you'll be set. Don't want her to deal with many status effects but want her to keep good buffs? Throw on Hades + Added Effect on her Armor, and you have yourself a weak Ribbon. But what to do with those other three double slots? Hmm, oh I know, how does cheese sound to you? Throw on Planet + Quadra Magic, Planet + Mp Turbo, and Planet + Magic Counter, and BOOM instant Chedder. You could sub out Magic Counter for Mp Absorb for even better cheese, but at a risk of crashing thanks to an engine bug that I've recently found out about. I also liked subbing Planet + Magic Counter with a Summon + Quadra Magic for that extra damage in case I run out of Quadra Magic on Planet, or if I wanna start a fight with a BANG. After all that magic her health is looking a bit low, throw on that HP Plus you have, and she'll have HP that is a bit more than Tifa's
Unfortunately you can't leave Cloud out of everything. Sometimes you get stuck with him, and that's alright. Him and Aerith are easily tradable. His magic isn't as high as Aerith's, but it's good enough for what you need to do. Throw on an accessory that boosts his magic a bit if you feel it's just not doing enough.
With this set up, I was able to take down Pinnacle without an MP Turbo'd KOTR + Great Gospel, and without switching party's. Tifa straight brute forced through the healing doing 9999 per 4x Cut hit while Cloud Quad Ultima'd the head down whenever it came up. I was about to start setting up to switch to another party when Pinnacle used Stigma hitting everyone. Next thing you know, Barret Countering 3 times doing about 3k each, Cloud countering with Ultima doing about the same, then Tifa Countering about 5 or 6 times doing 9999, then following it up with 4x Cut right to the jaw of Pinnacle knocking him right out! So good.