The final boss is probably easier if you take the time to develop a good strategy - but by the time I got through the Palace Invasion, I just wanted to finish the game already. The boss has multiple forms or "stages" that he cycles through (and I don't remember the exact order, but here's my best guess):
1. Just the boss. If memory serves me correctly, when there's only one "boss" on the screen, he's basically invulnerable. So don't use your djinn when he's alone, or you're just wasting your points.
2. Boss + friends. He summons 2 "helpers" who pretty much look like him but aren't nearly as powerful.
3. Boss + snakes. He summons a boat load of snakes, which gang up on you and can poison you. A lot.
4. Boss AS snake. The boss transforms into a giant snake and the screen goes black. The snake moves across the screen left to right, then right to left, and if he catches you he starts to crush you and does some damage, and then when he releases you, you're poisoned AND cursed.
So....
When it's boss + friends or boss + snakes, use your Shadow Army, plus X-Slash and Moon Blade. Watch out for poison and have plenty of antidotes on hand.
When it's boss AS snake, try to outrun him, but if he catches you, use Ziv or another djinn with a good attack to get the snake to release you.
When it's just boss, try to avoid him and wait for him to summon friends or snakes.
You'll want to have a lot of antidotes and paper amulets, and a ton of Huge Red Potions (and Huge Blue Potions to a smaller extent) - and MAKE SURE you watch your health closely (which can be hard to do when you're surrounded by 15 snakes, but is absolutely essential.)
I wish I could offer more, but as I said, action RPGs are not my strong suit. Hopefully a few other players will be able to offer their strategies as well, and together we can help other members.