So it's certainly possible to defeat Tock, the secret final boss, without using any max healing potions. I beat it on my first try at level 90 with Tier 9 armor, helm, and weapon. I don't think it's possible to get to the fight without the Tier 9 armor, but be sure to bring a Tier 8 weapon to Dionysus and Apollo and a Tier 8 Helm to Tenshi (in the Nymph Sanctuary in Hell) to fight some side-bosses to get your level 9 stuff. Use Nyangoro's apartment or the ice tower to farm for cash, although I don't think the Mythril Tower is super feasible considering you need nearly 370,000 shekels to get all the T8 stuff.
At the start you can be fairly aggressive. Attack with Nyannagedon and Nyanyanistar when you have a lot of health, and heal yourself when you drop below ~5000 health. Keep your mana high enough that you can heal yourself 4 times at least (so keep 2000+ mana whenever possible) and do have a potion in reserve in case Tock decides to Quadruple Nyan you like 50 times in a row as she sometimes will do. Try to keep track of what "works" on Tock throughout the battle. Every three turns she switches her weaknesses and strengths. It's not as simple as "weak to physical and then weak to magic" but that's a fairly good rule of thumb to follow at the start.
Eventually, Tock will say something like "Oh, this is an excellent form." This is when the battle changes. Tock's forms become more varied and complex. Sometimes she is utterly immune to damage. Sometimes she resists most magic and has high evasion. At this point, switch gears and go to full defense. You should be attacking pretty much never. If you're hurt, heal. If you are on full health, use Halcyon of the Mind. If you have full health and mana... just guard. Tock's forms are too varied to keep jabbing at them to try to figure out what does or doesn't work. Eventually, Tock will say something along the lines of "Oops, I made a mistake. I better switch again..."
This is your chance. If you've been keeping yourself healthy, you should have one or two turns to attack her. The quote indicates that Tock has effectively dropped her defenses, and your attacks will deal massive damage. I suggest using Nyannagedon now. If she doesn't get lucky and dodge any hits, you're looking at an easy thirteen or fourteen thousand damage. If she doesn't hit you particularly hard, do it again the next turn, and then go back to defense to ready for the next opportunity. It'll take a while, but it's good and reliable.
Also, Halcyon, awesome game. Thank you so much. I wonder why more games don't follow by this game's example and not give you a game-over screen every time you find a vore scene. One tiny typo--Tock misspells "deity" as "diety" in her last line before fighting you. Also, nice touch with removing the "surrender" button during the fight atop the Mythril Tower so we don't go killing ourselves only to find that there's no vore scene