Only after beating the game’s storyline are you able to find the extra, brutal boss known as Julius.
Once you have beaten the game and saved a "Game Clear" file, head into Traverse Town as either Sora or Riku - it won’t matter at all, except statistically. Have Sora land in the Post Office and go north an area, or Riku land in the Back Streets and go into the Fountain Plaza.
There in the Fountain Plaza, you’ll watch Julius rise up from the sewers below. What a big boy…
Credits for this strategy go to Jaysen Stoudt. This amazing and in-depth strategy is exactly what I used at about Level 55 to win with both Riku and Sora not once, but TWICE. It is amazing. I mildly edited it for spelling and minor suggestions, but it is his content.
Keyblade: Whatever your strongest one is! (I suggest that you beat all the Special Portals for the End of Pain, or even better, beat all Secret Portals for Unbound. Alternatively, just achieving A-Rank Dives for the magically-inclined Divewing, or clearing all Flick Rush Cups for it’s strength-oriented Sweet Dreams, can be benefical.)
However, all of these can be very time consuming to obtain, so you can always just use the strongest story-given Keyblade, the Counterpoint, but you’ll be in for a long(er) road. Strength-wise, the Unbound is your best bet.
Command List Suggestions: - Balloonga - Firaga Burst and other far-reaching, quick magic! - Curaga x2 (at least!), with one of them set to your shortcut - Quick, powerful close combat commands
On the matter of the final item, Jaysen used Shadowbreaker, Icebreaker, and Ars Arcanum sequentially for Sora.
For Riku, I’d recommend switching the Sora-only Ars Arcanum for Limit Break (attack that is more powerful as your HP is lower, which happens often here) or Drain Dive (for the HP-boosting benefit, albeit a slight benefit), or perhaps even another Icebreaker or two, if you can get more.
Action Command List Suggestions (from KeyBlade999):
High Jump - Dodge Roll (Slide Roll on Sora) - Air Slide - Aerial Recovery - Double Impact - Doubleflight on Riku; Glide/Superglide on Sora - Sonic Impact jointly linked with Sliding Crescent or Sliding Sidewinder - Wake-up Block jointly linked with Counter Aura or Counter Rush - Shadow Slide jointly linked with Shadow Strike (if using Riku) - All of your Flowmotion Commands
The Fight Itself:
Julius has a moderate amount of moves:
Left Punch (from Sora’s/Riku’s view): He’ll wind up his arm before quickly punching downward at you. The best way to dodge this is by sliding away from him diagonally left or right (be careful NOT to accidentally enter Flowmotion off of anything, as that’ll likely get you hurt in the long run).
You CAN block this move as well and counterattack, but it’s easy to be fooled into blocking and get attacked by one of his unblockable moves.
Right Uppercut (from Sora’s/Riku’s view): A blockable and quick move that knocks Sora/Riku into the air. If he hits you, he’ll probably follow it up with another one and then either his Body Slam or Left Punch.
You can try to get out it by Air Sliding away, but it is highly unlikely that you will be able to get out of this combo once he hits you, so just try to avoid it with the same diagonal dodge as Left Punch, or perhaps some blocking.
Body Slam (unblockable): Julius will fall to the ground, damaging you before quickly getting back up. Generally only a close-combat move, if you stay far enough away, he won’t be able to touch you with it, so just diagonally dodge.
Elbow Drop (unblockable): Julius will flail his upper body twice before jumping across the screen to try and elbow drop you. If you diagonally dodge, you can use the little bit of down time to either Curaga or hit him with far-reaching magic.
Fists of Fury : He’ll smash his fists together twice before he proceeds to alternate five punches while quickly moving across the screen towards you, culminating in a final spinning punch. Dodging this is possible, but not the best plan.
This is your moment to counter! It is the easiest attack to predict and time, so block when he gets close and counter immediately. If you wait until after the first punch or two, you should avoid all damage.
Grab and Screw (unblockable): Julius sweeps near his feet to grab Sora/Riku and pulls them to his face before laughing and spewing who knows what into your face. The Screw portion of the attack in named because this spewage then causes three status effects, in random order:
Blindness (area of view is reduced to a medium-sized circle in the middle of the screen that sometimes causes you to un-lock-on), Confusion (controls are all reversed, right goes left, left goes right, etc.), and Poison (your HP quickly depletes for probably about 10 seconds), assuming you lack the Block Spirit abilities.
Obviously, you want to diagonally dodge this move, but if you are caught, dash away AS SOON AS YOU HIT THE GROUND! He’s likely to Body Slam, Right Uppercut, or Left Punch you.
Charging Bull (unblockable): Julius will run after you, and then turn around to do it again before screeching to a halt. This attack is really easy to dodge and should be another great opening to use that far-reaching magic, Curaga, or even Balloonga once you are out of harm’s way.
Butt Pound (unblockable): If you get too far away, get on top of a building, or even just enter Flowmotion, Julius tends to respond by magically leaping to the tallest building, often the southernmost ornate building, before continuously butt-bouncing onto you. As of now, I have never been about to fully dodge this attack while on the ground.
However, if you can get high enough using the surrounding outer buildings with Flowmotion (and keep moving while maintaining that height!), you should be able to dodge it. If not, a well-timed Curaga, if you have Leaf Bracer, can avoid most of the damage as well as heal you (but be prepared for a follow-up attack to hit you as the animation finishes).
Now that you know the moves, it’s time for the strategy. All-out flailing is useless in this fight and will get you killed very quickly. (I should know, that’s my general play style in this game, and after trying it once, I knew I would have to change it up.)
The best way to beat this guy is to be patient. Dodge ALL of his attacks (a diagonal dodge is best, or blocking with counterattacks where possible) until he tries Fists of Fury. Block it and use your counterattack to do some damage while avoiding taking any.
Julius, after a certain amount of hits, will instantly stop what he’s doing (even in mid-attack) and be stunned for a second (two counters or a counter and another command are usually enough to do the trick).
The best way to capitalize on this is by countering Fists of Fury, then immediately using your quick, close-combat commands (e.g. Icebreaker, Ars Arcanum, Shadowbreaker, etc.). Normally, this will stun him long enough for you to dodge away before he attacks again.
Every now and then (I can’t find an actual pattern to it) Julius will randomly just stop and sit there, it’s only for a few seconds, but if you can capitalize on it with some magic, you can deal some extra damage to him, or even just heal (his wide array of close-combat attacks make getting to and hitting him close-range impractical).
If you just bide your time, be patient, and react accordingly, then you will be able to easily get him down to half of his health. What?! Half of his health ? Why there? Easy, this first half was the easy part. Now it gets hard. As if it wasn’t hard enough.
As if Julius wasn’t made to be the most annoying and probably hardest boss battle in Kingdom Hearts history (with the definite exception of Mysterious Figure and possible exception of Vanitas’s Reminant), at half-health he seems to get the powers of Zeus as well.
After calling down a barrage of lightning that will instantly kill you if you don’t keep moving, Julius gets a power boost and a nasty little shock added to his attacks.
His Right Uppercut and Left Punch now have lightning come off of his fists toward you. While these are technically dodgeable, it requires precise timing and a lot of luck.
Lightning Mode Julius has one final surprise for you: he can disable whatever command you have currently highlighted so that it take four to five times longer to reload (effectively taking it out of play for the rest of the fight, more than likely).
YOU DO NOT WANT THIS TO BE YOUR CURAGAS SO BE EXTREMELY CAREFUL!!! This can be avoided on restorative commands almost 100% of the time by opting in the Config menu to have you skip over them in the heat of battle.
Luckily, this power-up eventually goes away, after what seems like an eternity. The best way to deal with Lightning Mode Julius is the same way you deal with Butt-Pounding Julius: Flowmotion high in the air and out of his reach until he returns to normal (you should be able to tell the difference because his fists will no longer have lightning around them).
As for avoiding the lightning, the only technique I’ve been able to effectively use is a Curaga with Leaf Bracer on. Although, you will likely be hit by some of the lightning still and/or his next attack, so be careful, and get to the buildings as quick as possible (if he’s using Fists of Fury, you can still block and counter with no damage taken, but be careful of his other attacks!).
After he returns to normal, just wash, rinse, and repeat until he’s defeated. He will every now and then return to Lightning Mode, so be ready to outlast it at any time after half-health.
If he only has half a health bar left, it MAY be better to try all-out attacking if he starts to go Lightning Mode (that’s what I did to finish him rather than facing that wrath again), but attack at your own risk; you don’t want to get that close and lose it all!
Even with all these strategies mastered and perfected, this is still no easy battle, so be prepared to die A LOT before winning, but the reward of the Ultima Weapon Keyblade is well worth this frustration (although, unless you fight him again, you probably won’t really need it)!
Note, of course, that winning with Riku doesn’t unlock the Ultima Weapon for Sora, obviously, and vice versa. You have to beat 'im twice, once with each character.