I suggest if you are having trouble to level up. Xemnas is not at all hard, and you're probably just going to need some power levelling. The best place to level is in The World That Never Was.
You should probably also consider leveling up your drive forms so you can use abilities like Double Jump, Glide, etc. They are pretty useful, and will definitely help you out.
First, in order to find him, you will have to go to Agrabah after you finish Hollow Bastion for the first time. Then, go to Alladin's house and there, you'll find Carpet. Talk to him and the game will say that Carpet is excited and he wants to go out. Then answer yes to the question and you will see a cutscene. After the cutscene, the battle commences. Now, he will stop you from using magic. He will have dark balls in his hands which you have to hit to get to the main body. When you get both spheres, he will fall down and his head will kinda rip off his shoulders. Start hitting the head immEDIATELY!! After that, a dark force field will surround him. Strike it with magic. Thunder/thundara/thundaga works best. Then, head-smashing comes again. Now is the hard part. He will start flying around and spinning horizontally and vertically at you. One or the other. When he spins horizontally, jump and glide over him. DON'T do it if you don't have High Jump equipped(you get it from Monstro). Just Dodge roll him. When in vertical position, dodge roll left or right. Whatever seems more appropriate. Do all that and have elixirs and potions equipped in the item slots. You'll beat him easily.
Go past the new opening (where the rock wall was but disappeared during the fight with Clayton.) To the left, there will be a ramp made of rock with plants on it. Jump up this ramp and onto the ledge. Go to the left and you'll enter an area behind a waterfall. Climb up the rocks to the very top and you'll come to a place called ????. Enter it and you'll seal the keyhole.
Keep healing and attacking. If you still have trouble, try leveling up somemore
If by 'Atlantis' you mean 'Atlantica' Then in KH1 you defeat Ursula by finding her in her cave and zapping her with all the Magic you have, in KH2, you simply do the musical tasks!
All you have to do is attack his little fire hair thing. Ragnarok does a lot of damage. Also, you should have cure/cura/curaga.
Beat the trickmaster and save Alice (you find out there is no one to save)
Focus on defeating Clayton first, because he can heal
Hit him in the hand or head.
Kingdom Hearts 1: Beat Cerberus in the hades cup Kingdom hearts 2: Complete the Pride Lands for the second time
You can't play Kingdom Hearts Final Mix after beating the regular Kingdom Hearts games. They are separated and needed to be bought if want to be played.
No, just beat Melifecent
Go back after you're done at Hollow Bastion. There will be a sea urchin. You have to mess with that to open the shell.
After you beat the Parasite cage once.
Kingdom Hearts 1: Beat Cerberus in the hades cup Kingdom hearts 2: Complete the Pride Lands for the second time
Play Kingdom Hearts 2 and wait until Kingdom Hearts 3 comes out. Hope that helps! =D
By attacking her.
You have to find lots and lots of heartless.
Hearts in Atlantis has 528 pages.
Fight.
You have to beat Ansem
No
Get Kingdom Hearts 2. That's what I did. Or you can try and beat sephiroth and all those extra things.
Hearts in Atlantis was created on 1999-09-14.
The Production Budget for Hearts in Atlantis was $31,000,000.
Hearts in Atlantis was released on 09/28/2001.