This optional fight with Vorselon is probably one of the hardest parts of the game, if not the hardest. You'll have to first beat the game, then go back in the same save (not challenge mode), collect all 40 Zoni, and he'll become available.
There's a few things you have to walk through before you fight him that aren't too hard. You'll go use a turret to kill enemies and get 2 battery bots, then swingshot around a room and use the omnisoaker/wrench to traverse a second. After that, you'll fight Vorselon.
Early on, I liked to use the Groovibomb Glove to keep him in place for a bit, then while he was stunned by that, I'd hit his eye weak point with the Plasma Slayer (sniper). This does great damage, and it's pretty much the only way you'll be able to effectively use the Plasma Slayer's ammo. Tossing the Rift Inducer right above him while he's stunned is a good idea too. After you've run out of that, start bringing Zurkon out. The Doom Blades work well to do lots of damage until he starts disappearing at around 25% health.
At this point, the Negotiator/Judicator (rocket launcher) works wonders to get in lots of damage in the little time that he's actually visible. If you run out of ammo on that, Doom Blades still work fine, or you can throw out charged Dynamo of Devastation shots (SIXAXIS steered energy ball). Both are good options.
Of course, you should avoid the blue sections of the floor when he throws those out. They'll rack up damage on you that you can't afford to take.
Generally, using your zoning weapons will help quite a bit. Tesla Spikes are great for hitting him, but spread them out so they don't become useless as soon as he moves away. The Rift Inducer too can be great when you know he's gonna be in a certain area for a bit, if you didn't use all the ammo for it when you used the Groovibomb Glove's ammo.
For defensive options, he's got a number of attacks, so you need to know how to deal with them. Here's how to defend against each one:
Vorselon shoots a rapid fire, three lane blaster at you -- Double Jump and hover, try to land between the lanes of fire. DO NOT TRY TO SHOOT AT HIM. You'll get hit if you get greedy and try to get more damage in while this attack is going.
Vorselon throws a spinning X at you on the ground -- Hit Vorselon with the Mag-Net Cannon to interrupt this so you don't have to dodge it. There is no easy way to dodge it otherwise.
Vorselon throws multiple homing energy bursts on the ground in a pattern -- These aren't too hard to dodge, just try to jump over them when he throws these at you. They should hit each other and disappear.
Vorselon tosses multiple lightning bombs in the air -- Your best bet here is to just be reactive. They aren't too hard to dodge, but if you realize that you're on top of a target where the lightning is gonna hit, move to where there's less targets on the ground, but do it fast or else you'll get hit.
Vorselon disappears, and when he reappears, throws a melee attack at you -- Long jump away from the direction he reappears in. You're almost guaranteed to not be hit if you long jump, because you can quickly get out of his melee range.
If you know how to deal with his offense, you won't get hit too much and the fight becomes much easier.