For the most part, Number One difficulty isn't that hard, just a few minor frustrating parts and 2 irritating boss fights. If you made it to this difficulty, then you shouldn't have an issue with most of the platforming and normal fights. That being said, I only really got stuck on Justice, but it was for a good long while, so that'll be my highlight for this solution.
Fighting Justice on The Number One difficulty can be a huge pain, so here's the best way that I found to beat him.
When the match starts, run around dodging the bullets and focus on getting a single hit on him. Don't forget that you can click in the left stick to sprint because I absolutely did for about an hour and it really helps get close and away. He will then dodge into the ground and do about a 30 second speech. At this point, one of 2 things will happen. He will either
(A) Have a puddle of darkness under you, from which he will teleport undo you and try and surprise you from behind
(B) If you are in a far area and he cannot find you properly, he will not do the darkness puddle, and will then spawn in the ring of trees and slowly saunter over to you.
Hide behind the trees as best as you can until you are able to get him at medium range which will cause him to run towards you. The most important thing to do is to hit him with a repetitive X combo whilst not using any focus attacks. It sounds stupid but what he will do is he will knock you back and do his fire 5 bullets, pause for a second and then fire the 6th bullet attack. Block the first 5 and IF YOU CAN, return the 6th bullet. Regardless, run up to him and hit the X button a few times until he does 1 SINGLE SHOT from one of his hands at pretty much point blank range. IMPORTANT! - After this move, the game/Justice seems to have an issue with his attack interrupt ability. What I mean is immediately afterwards, get up in his face and continuously tap the X button and he will block the attacks occasionally, but he won't be able to interrupt the onslaught and he will eventually either go into darkness or he will cause the first weapon clash. After which, a small cutscene will happen of him twiddling his guns. Hold block the whole time in case he fires his 5 to 1 attack after the cutscene. Majority of the time though he will run at you and your main objective will be to hit him while he is still trying to reach you, using the XXXX combos. He will continue to do the same loop over and over. Another important thing to note is that when he seeps into his darkness and when he does a mini-cutscene, the game will act as if no enemies were around, and it will cause all the blood to be off your shirt i.e. your health has regenerated. After a few times, a cutscene of you cutting his arms off will occur and then he will knock you away with a barrier.
First Mutation Justice:
The form is very easy and shouldn't give much issues. Try and stay in range of Justice, just for sheer vantage point, because you can watch his arms sink into the ground which is pretty much his only attack in this form. His pattern is simple as all it really pertains to is him sinking his right arm into the ground and popping it out on the other side of the barrier, followed by the single left, and then finished with both, sometimes twice in a row. All you have to do is run in a left to right fashion as his arms come out and then when you get both the arms out at the same time, hit them both with a horizontal perfect slice. The arms will get cut, he will regenerate them and then repeat up until what I believe to be the 4th time when he will immediately pop out about the arms repetitively. I got hit by about 3 of them and then was still barely even damaged so this shouldn't be an issue to much because he will then do the same pattern he used the rest of this form. The next cutscene will trigger and he will now have what look like 2 giant 3 pincered claws? Wtfe
Pure Mutation Justice
This form is a little more annoying than the 2nd because he actually moves around, but he is still fairly easy if you have a gameplan. He will basically just run around and try and get close to you and then he will do a large radius arm swing. It's easy to dodge if you just move to the right of him and then immediately follow with a focus/perfect vertical slice, or sometimes even 2 while he is recoiling and then run away. You can also if you feel like stalling him so you can get your focus back, hit him with the B button combo and then after he gets hit with about 3-4, interrupt your combo with a jump to avoid his knockback. This is his main attack for this form so just repeat until he goes into the trees and swirls his hands. He will now run around like an idiot sometimes when you do the vertical slash to catch you off guard, and the head at you with the swing attack. I like when he does this because it gives you time to have your focus attack regenerate. Sometimes he will do a strange and slow tornado like motion with his arms out and is fairly easy to avoid. Just keep the same move going and this should cause him to go into a cutscene where he walks on his hands... Now he will start shooting waves of darkness at you which do a small amount of damage, but for the most part mainly serve as a way of knocking you down so try and avoid these at all costs because if you get hit by about 5 of them, this fight is going to be more annoying. Try and get close enough where he will do any melee attack, of which are very easy to dodge. The two which you want him to do are either
(A) Swing his left arm, then his right arm, then pound the ground
(B) Pound the ground making shockwaves while you are close to him
Both of these have semi-large arm radius's for him but afterwards he gives you a good amount of time, about 1 second as he is finishing his swings, and 2 seconds of leeway before he will try another attack. I found that he likes to eat perfect vertical slices this form, and circling to the side of him as he is attacking always kept me safe as he was attacking, and gave me ample time to move in on him as he finishes. This form while is fairly offensive, only takes about 2-3 vertical slices for him to die.
I hope I helped at least one person, because I tried everything and this was the only way I could manage.