It's kind of important to know which "final" boss you're talking about 😉
There's the God King at the top of the tower, he's not too bad. Practice dodging his attacks, and parry the easy ones. If you find an attack difficult to dodge, use the block. The god king is NOT the final boss!
Then there's the 3 deathless ones that you find down in the basement (you have to defeat the god king and get the infinity blade to even get to these guys). They're hard. The golden one especially.
Beat them, and a door opens, leading to the robot. I used the same strategy as the god king here - dodge or parry as much as possible, block the rest. Kill him, and the REAL final boss appears. Beat him, and the game really, finally ends - no more "20 years later" crap. It does let you restart the game and keep your level ups though.
Most important strategy I reckon is to use magic. Level the magic up at least as much as your attack + health (I actually found block the least useful). Then equip a ring with healing. You can heal yourself regularly - if you've leveled up enough, for the final bosses you can heal after almost every attack. I think it would be near impossible without that, because if they hit you just once you're nearly dead!