This is the team I used. The key components for the fight are a strong healer, someone to lower fire resistance, a magic fire DPS and magic damage material on all, as well as a diamond sigil breaker.
First, lets go over Aerith. On the subs, I’m running seaside collar to boost HP, close to 10k HP should be enough, a bit more won’t hurt. I also use red’s headphones for the boost Mdef all, as well as the ice resist, which is a good R ability for everyone in the ex battles, as well as floor 21. Finally, fairy tail is on the back to boost heal as close to 3k as possible. It can be used as main if you don’t have either of the main choices I’ll go over below
Going for breath of the earth is the best choice for the limit break, as it charges a bit quicker than healing wind.
The main weapon choices may seem a bit whaly, but I’m 100% F2P, I managed to get OB1 kamura wand during the last collab, which helps with increasing magical defense a bit more, and Bahamut Rod has two key things that help in this battle: the increased magic attack for the whole team, and the 3rd slot support materia, esuna All. Running an esuna silence materia there, and casting it at the right time, can help you avoid getting silenced each time Shiva stars a new cycle.
If you didn’t get the bahamut rod, or bahamut robe, you can still use Aerith, the chocobo suit is a good alternative for survivability.
If you don’t have a good Aerith, you should definitely try using Matt, however, it’s a bit difficult to manage having good group heals and a magic defense buff for the whole party with him while still reaching the 12k HP and 3K heal target. If you don’t have either well built, this battle calls for a good enough healer, and Tifa as a distant third party healer won’t cut it here, probably. Barrett is also a choice, since he has a single target mdef buff, but his heals are on the low side.
The two choices for fire DPS are Tifa and Zach, which one you go for will depend on whether your Tifa’s Shell Knuckles or Zach’s Stream Guard are better leveled, but on base, Shell Knuckles do more damage. A physical DPS doesn’t really work, as an example, I’ve got Cloud’s Skysplitter OB10 lvl 120, and I can get about level 8 on the fire damage increase R ability, but it barely makes a dent to Shiva’s ice gauge.
While I can get Tifa to around 8K magical attack, that would require running a completely different setup, forgoing the diamond sigil break, and a lot of much-needed survivability. I ran out of mythril to boost my Shell Knuckles all the way to 110, but nothing more than that should be needed. If you’ve OB6’d them, and you have the fire arcanum gear for her, she’ll be way stronger on this fight. As for the secondary, its bahamut’s knuckles, don’t mix them with bahamut’s fangs, which are the anniversary limit break banner ones, these carry the diamond sigil breaking c ability, as well as some HP and magic defense, really good for this fight.
You can personalize the back a bit, as I’m running OB10 crystal gloves, which I wouldn’t expect too many people to have gone for. Flower vase is really good , it brings up a lot of boost magic attack R ability plus magic potency, which works with any magical attack, however, running a high boost magic attack and boost fire damage combo would probably make it a tad easier on you. Crystal Sword is being used as boost magic attack is already at 7 with this combo, so no use on running another heavy magic attack weapon.
Now, the final, and most important team member, your controller (or debuffer, if you want). Red XIII is a no-brainer here. He requires some investment, but it’s well rewarded, as he can help a ton on any battle.
Red is not here to do damage, he merely needs to survive, and continuously lower magic attack, by using Junk Collar’s Frenzied Fang, important whenever a big attack comes up (especially if you can’t use esuna all silence at the beginning of the battle) and then lower fire resistance by using Growing Ivy. This setup is great as it is just because Frenzied Fang+ lowers three levels of magic attack in a single go, and Growing Ivy is a cheap skill to cast, which will help you lower Shiva’s bar faster, as well as getting tons of damage while she’s stunned. With this current setup, Tifa’s Spinning fervor gets to hit for about 100k damage while on the stun phase.
As for Red’s sub weapons, having Ifrit’s sword may be enough, you can choose to swap the other two for as much magic defense or HP as you want, but this setup is kind of the best I had after using pretty much the best on Tifa and Aerith.
As for the battle itself, each time Shiva generates a bar, she’ll start by silencing all the party. Using a physical magic attack debuffer is the best trick you can learn, but, if you have bahamut rod, go for syncing up whenever she raises her hand to cast esuna silence all on the party for 3 ATB, and that will make it easier. You’d STILL want to debuff Shiva’s magic attack, and use defense stance when the attack goes off, and you should definitely heal as much as you can after the attack, as you’ll likely be close to death after most of them any way you prepare. Work on keeping her magic attack and fire resistance down, hit all you can with your fire DPS’s main attack to get her casting bar lower, go for the interruption whenever she starts her sigil phase, you’ll have to manage heals manually during all of this, as well, and while both EX 1 and Floor 21 of the current tower event are just repeats of these three phases, EX2 Shiva will cast two silences on a row around the third cycle, normally whenever she’s around 25% health. I haven’t managed to handle this part well enough, as it takes a bit getting to it, even, but it should be doable with a similar setup, and maybe some tiny adjustments. Hope this helps you clear floor 21!