Preparation:
1. Get 2300 HP via leveling or the shop in the forest (I was at L.24 and spent all my money on HP Seeds)
2. Equip Murasame or an Axe, and the pendant that prevents armor break. Other equipment doesn't matter, but more defense won't hurt.
Murasame is obtained by selling used panties to the guy in one of the towns. Sell one of every type then talk to him again after obtaining the Holy Sword late in the game. The pendant is found as part of the story.
Battle:
At first you just need to survive.
1: Cast Saint Plus (24TP, Large HP regen)
2-3: Fully heal if <~1500HP, otherwise Defend.
4: Cast Saint. Hope you survive.
5: Defend.
With 2300HP the odds are about even.
Keep the regen effects active with the following pattern...
1: Cast Saint Plus.
2. Defend
3. Cast Saint
4. Defend
If you have more than ~1900 HP remaining after casting a Saint spell, do one of the following:
- Murasame's 40TP attack skill. Or Axe's 30TP attack skill.
- If MP is low (< 50%) use the 50TP MP restoration skill.
- If MP is dangerously low you can use an Elixir to negate the HP requirement.
Then defend, cast the other Saint spell, defend, then resume the pattern.
Repeat until the lesser enemies die. If using an axe, kill the left enemy first.
When the boss is alone:
1. Cast Saint Plus.
2. Cast Saint.
3-5. Murasame/Axe 30TP attack skill. Or 50TP MP restore.
Nothing else will do any damage. Repeat for like 30 minutes. It has 99999HP.