Yeah that's the one. I was very close to failing it at the end. The game deeply wants you to use magic attack spells on that boss in later phases but it just feels wrong to waste MP on them considering the OHKO moves it has, forcing you to res or Curaga someone back from the brink. Pure horror tbh
Personally I didn't find Arsenal that tough if you understand how the mechanics work.
You can avoid most of the damage he pumps out by hiding behind the pillars during the first phase, and you really only need to use spells to heal / raise. You can take the early phases of the fight pretty slow since his damage output isn't that severe. Just block when he uses homing lasers and stay behind cover when you aren't interrupting his Primary Fire.
When it's using Primary Fire, have Barret use Focused Shot or Aerith use two Spirit Drains to disable the gun. It will then use Primary Fire again; repeat the process and Red XIII will use Stardust Ray (I think it was anyway) to pressure/stagger the boss/barriers. Get the barriers down and it'll start moving around and using Charge, which is pretty easy to avoid.
Essentially repeat the process from there. When he targets a character with Particle Beam (not sure of name), hide them behind a piece of debris while the other works on attacking. You can also continue mashing attack in cover to build ATB even if you aren't hitting him. After the cover is all destroyed Particle Beam will guaranteed kill the targeted character unless they have Reprieve. Just Curaga or Arise them depending on which happens.
The only tricky part is the final phase, where he traps you between flame walls and aims a big Particle Beam at your party. The first one can be avoided with one last piece of debris, however the next one will kill you if you don't finish him off in time. Fill your ATB as quickly as possible and use Maximum Fury / Ray of Judgment to finish him off. It took me I think three Maximum Furies and two Rays of Judgment, but it worked. You might also save your summon / LB for the final phase.
It will help if you bring your highest ATK weapons, and you don't need to use elemental materia since she shifts resistances during the fight.
I've heard people having trouble with Hell House too, but I thought it wasn't too bad. Main advice, have Aerith cast Bioga with a strong MATK weapon that has 10% debuff duration. Otherwise, try and keep Cloud flanking the house to avoid getting sucked in by Hospitality, Triple Slash the tonberries it summons (they have low HP), and use Arcane Ward to double cast whatever element it is currently weak too. Again, don't bring an elemental weapon because its resistances shift. The Sweepers (or Cutters? I forget) it brings in towards the end can be burned down pretty quick.
Also, if you're really struggling at any point, you can try and jump ahead to Chapter 17 and finish up the VR challenges to get Gotterdamerung.
Main advice I have for the Top Secrets fight: Bring Lighting + Elemental on Cloud's weapon (for Leviathan and Pride and Joy), put Fire + Elemental on Aerith's armor (absorb Ifrit's attacks during the Bahamut fight). Bring Fire spells on all three party members to deal with Shiva effectively. Definitely bring two HP Ups and one MP Up for each party member. Give Aerith Magnify + Barrier. Have everyone equipped with Healing + Revival Materia. Give everyone else either Chakra, or give Cloud and Tifa Chakra and Aerith Prayer. For the remaining slots, ATB Assist, ATB Stagger, Steadfast Block, Skill Master, etc. on party members of choice. First Strike isn't that useful in prolonged fights since it's a one-time benefit. Might consider bringing Ice so you can nuke down Ifrit, but I didn't find it necessary. You may want to bring Headbands so Shiva doesn't sleep you, but it's not mandatory.
Shiva is a pain in the ass, try to nuke her down with Fire/Fira using Arcane Ward, then try and time your ATB so you can maximize damage during Stagger. Maintain Manaward (don't need physical reductions, so don't use Barrier or Manawall) and heal as needed. Try and avoid Icicle Impact and when she does the big ice block drops, you have to run to avoid those or die. The annoying thing about this fight is her Ice Familiars. She "consumes" them to absorb fire attacks or to bolster her own attacks. When she runs out, hit her with Fire. I'm pretty sure you can just spam weak fire spells to eat them up quickly, but depending on RNG it seems that sometimes the AI will just keep reapplying them, in which case you might want to just retry the fight. You can interrupt Diamond Dust by staggering her, but if the timing isn't good for that, just Manaward and heal up before and after.
Fat Chocobo is easy, pretty much identical to the first fight with him. Maintain physical mitigation with Barrier, then basically just work him over to build his stagger, then unleash everything you can on him. I like to use Chi Trap since he just sits in it. When he uses Wark Wark and starts rolling around, just hold block. And roll out of the way of Boom. Very easy to manage. For the adds, just spam Triple Slash to quickly kill them.
Leviathan is pretty much a pushover. For this one you might want to use Manawall, though his physical attacks aren't too common or powerful so Manaward should be sufficient. Get up close, pile on damage as much as you can, won't be long before he summons the water spouts and starts flying around. You can basically use this phase to replenish MP and just whittle him down. Avoid the spouts, use chakra/prayer to keep up, maybe have Aerith lob Spirit Drain, basically just wait for him to fly low to the ground and then work on staggering him. Deal with Tidal Wave the same way you dealt with Diamond Dust, either Stagger or Manaward + Heal.
Bahamut is without a doubt the hardest part of the fight. Counterstance is especially good since he does quite a bit of melee, just be aware that Umbral Frenzy can't be blocked or countered, or even reliably dodged. Most of his attacks are pretty hard to avoid, frankly. Maintain Manawall (both phys and mag mitigation), you probably want to try and get Regen up to help take pressure off healing, too, but it's not mandatory. Also nice to have Aerith setup a Fleeting Familiar on this one, since it lasts such a long time anyway and helps a fair bit. Work his stagger gauge, especially when he stands still to countdown; Chi Trap is great when he's stationary. When the countdown is done you want to be ready for Mega Flare, so make sure you have ATB to spare so you can heal, Manaward, and re-heal.
At 50% he summons Ifrit. It may help to let him cast the first Mega Flare before you take him down to 50% so you don't have to deal with Ifrit at the same time, but it's still manageable if you don't. When Ifrit appears, focus entirely on him and bring him down ASAP. He doesn't have a ton of health and staggers pretty quickly from Focused Thrusts / Blizzaga / Shiva. After Ifrit is gone, the rest of the fight is just more of the same, so rinse and repeat and you'll win.
Pride and Joy (aka Proud Clod) is pretty much a victory lap. Stay behind it as much as possible, avoid its heavily telegraphed area attacks, damage its arm until it lets go of party members it catches, or be ready to revive them when it kills them. There really isn't much more to it than that.
