I remember playing the original KH II and beating Sephiroth around level 40. Now during the final mix I'm level 67 and I can't beat him no matter how hard i try. What am I doing wrong?

Can we just take a moment to appreciate the irony that a member named the BaneofSepiroth is having trouble beating Sephiroth...

What difficulty are you on? What Growth Abilities do you have?

 

Also, Sephiroth is not so easy an opponent that you can just blindly rush him and mash X and expect to win. If you figure out when you can Guard/Reflect, do so. It'll be easier if you can read his movements.

 

You also have access to Trinity Limit and, because it's Final Mix, Limit Form.

First of all, congrats on beating Sephiroth in the original KHII at Level 40!  That's seriously impressive!  I didn't go near Sephiroth till I was level 99, so yeah, you've earned yourself quite the hefty accomplishment!  

 

And well, my strategy is blocking his attacks, look for openings to strike, and make sure to have lots of Ethers and Elixirs for when he casts Heartless Angel on you! And well, yeah, just use Trinity Limit and Limit Form, and boom, there you go!  Of course, that's my strategy for taking him out at Level 99! So yeah. X3

If you had dodge slash equipped, remove it. That ability is terrible, especially against Sephiroth.

Also, what part exactly do you have problems against him? 

4 words: Limit Form & Trinity Limit

When he activates "Heartless Angel" i make sure to press the triangle reaction comand and then use berzerk charge to keep pounding away at him like the strat guide says, but then the guide says he SHOULD put those orb things around me, giving me time to heal. Instead of that he teleports behind me and kills me. I'm on proud difficulty.

And the name references FFVII not KH

I tried using limit form right after Heartless Angel and using Last Arcanum, but it failed because after I finished the limit he teleports and kills me

And I've maxed Valor, Wisdom, Master. My Final is level 5 and I havent found a good place to level my Limit form

Also, can anyone tell me a better place to level up my Final form? TWTNW is taking FOREVER

Berserk Charge... eh... not something I'd use since it cuts off access to your normal finishers. Yeah, you have Upper Slash, but I prefer using Guard Break and Explosion. I would personally recommend removing this and seeing what he does if/when he teleports after a finisher or two. It may help to make him a bit more predictable and easier to read.

 

Also, I'm pretty sure "Descent Heartless Angel" doesn't have a Reaction Command of any sort. Just whenever he does that move he opens with.

 

To Level Up Limit Form, you need to activate any of its Limits and complete it to get the Form EXP. The best way to do this is to go to Agrabah, and go to the Treasure Room where the Mushroom mini-game is. Activate Limit Form, then start the mini-game (so you don't accidentally Anti Form), back away toward the entrance and spam Sonic Blade until you're out of MP/Form, then go back to the Save Point and go to the World Map. That way, when you re-enter, you'll have all of your Drive Back. (Further, abusing this Mushroom is also how one could easily get Valor Form EXP and Summon EXP--the latter involves waiting for the Summon Gauge to decrease then smacking the Mushroom about so that it constantly increases and decreases, yielding the Summon EXP.)

 

For Final Form, a good spot is the Twilight Town Mansion Basement. Both there and TWTNW work if you take advantage of Firaga (except on Sorcerers, of course), as Final Form Firaga will shred through the Nobody mobs' HP. Having Oathkeeper and Bond of Flames equipped would help due to their abilities.