Actually in FF history, no boss has been as hard as Omega Weapon in FFV. Seriously, look it up, he could kill your party in one hit from the get go of the fight and you would only do about 200 HP damage, even with a maxed warrior with Excalibur.
Penance was actually easy to kill without even using the Zanmato Trick.
You just need to have your party at over 60K HP to avoid being slaughtered by his most powerful attakcs, 255 Defense, 255 Attack, 255 spirit and 255 agility.
Have accesories with Ribbon, Auto-Life, Auto-Haste and Break HP Barrier in the slots, and make sure that Auto-Life stays on during the fight.
Keep using Quick Hit during the entire fight, set up your Overdrives to Warrior so it builds up with the damage that you cause, go for the arms and then keep pummeling on Penance.
With this setup, I managed to kill him in 15 minutes, with no member of my team dying and Penance itself only getting 4/5 attack chances during the whole fight (As Quick Hit works by doubling your Agility stat for the next Turn Slot of your character, so with 255+Haste+Quick Hit, every new Turn Slot would have an effective 1020 Agility to it).
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
