Here's a written walkthrough:
1. You will need to beat the first 5 bosses, to break 5 seals, to fight the boss at the first castle you found.
2. I suggest getting the secret 4th armor, which is another achievement/guide.
3. I also suggest getting the best weapon, or best one you can get. The 4th one requires a special playthrough.
4. For my fight, I got all the skills/abilities, and 3 health potions. I think you can get 4, which would be best!
5. Fight the boss.
- The boss has a fire ball attack above head level, a fireball that falls on you, a lot of bricks that fall on you, he throws a melee weapon at you, he kicks at you, and he jumps on you - so there's a lot!
- I used the last ability you get to shoot fire balls at him constantly. Needed to use it twice to kill him, meaning you need health potions to heal.
- Dodge the attacks as best you can, but there's a lot of them - so it's tough.
Good luck!