This is the stage in the lower left corner of the map. You'll see two new enemies right away.
This is more of a trap than an enemy, since you can't destroy it. It sweeps a laser sensor back and forth across the floor. If you touch the laser, it will turn into a line of energy balls that will damage you. You can wait for the laser to eventually cut out for a moment, or you can dash straight through the laser when it's pointing towards you to trip the trap without taking damge.
Another crab enemy. This one fires two energy balls from the guns on his head. These ones don't bounce, though. If you damage it but don't destroy it, you'll destroy the guns, causing the Crablaster to surround itself in an electric forcefield. The forcefield will need to be damaged enough to dispel it before finishing the Crablaster off. It's better to destroy the Crablaster before it can use the forcefield.
The elevator here can bring you down to a teleporter that leads to Vile's stage. You probably still don't want to fight him, though. Clearing this stage will actually make Vile's stage easier. (Don't go down here if you don't plan to fight Vile. The elevator will disappear and you won't be able to get back up without impaling yourself on the spikes.)
Head right past some Crablasters and a Head Gunner until you reach an elevator. More Head Gunners will be guarding health on the way up. Head to the top and go left past the Trappers and more Head Gunners, then up past another Crablaster. Take the elevator. Don't touch the spikes on the walls.
Instead of going left from the elevator, continue to ride it up and go right. There's a heart tank on some spikes. Dash jump or air dash from the left wall to the right wall and slide down to collect the tank. Quickly jump back up to not touch the spikes and die. Return to the left wall the same way you came and head back down the elevator shaft.
Go left from the elevator when you can. To the left, you'll find some familiar enemies and a newbie.
These things sit under a glass dome until they're ready to attack. They attack by shooting electric orbs that split in two once they hit a surface. You can only damage them when they lower their domes. Head up the elevator.
If you have the arm upgrade and Gravity Well, ride the elevator all the way up to the left. Hop on the weird block and charge the Gravity Well. This will make the block rise. Ride it up to reach the Body Parts. This upgrade will reduce the damage X takes, as well as produce a forcefield that will further reduce damage for a few seconds. Useful! Head back down.
Go right from the elevator shaft to head outside for a moment, then back inside and through the empty room.
You'll need a Ride Armor for this. Any Ride Armor will do, but you'll need the Chimera Ride Armor from Blast Hornet's stage before you can use any Ride Armor at all. Head up and to the left to find the platform to summon the Ride Armor, then drop down the right to break the floor. The sub tank is down here.
Go down past the electrified walls and left past the Crablasters. There are also Caterkillers and Trappers to deal with. This path twists and turns a bit, but eventually you'll come to the boss door.
The annoying thing about Volt Catfish is that he'll constantly raise his defenses in the second half of the fight.
The Hard Way
A true fish out of water. He'll flop around, firing large, slow electric balls at you. These crawl along the floor and walls. He can fire spiked balls that stick to the wall, which he'll then shoot electricity at, causing them to float back to him. That's three things in one chain that you'll have to do your best to avoid. Use jumps/air dashes. Once he loses enough health, he'll start charging himself with electricity in the middle of the room, erecting a forcefield on either side. The forcefields will block your X Buster shots. He'll spray some sparks around and then charge towards you. The forcefields will disappear. This is your chance to attack, before he charges up again.
The Easy Way
He's weak to the Drill Fang. It won't be destroyed when it hits him, so with some luck, he'll jump and land in front of it and get hit by the same drill twice. The Drill Fang will make him flinch. In the first half of the battle, he'll always jump after getting hit with it, and not attack. Hit him when he's about to charge up in the second half of the battle. It will interrupt his charge and cause him to jump straight up in the air and land. You can then hit him with another one. Keep drilling him until he's dead.
This weapon creates a triangle of electricity that forms an electric perimeter around X for a moment before shooting electric beams straight up and diagonally down left and right. Charge it up to smash the ground and unleash a powerful earthquake and produce two electricity balls that crawl along surfaces to the left and right.
Defeating Volt Catfish will activate the elevators in Vile's stage, which will help you a great deal later. It also turns on the lights in Blizzard Buffalo's stage, which... does nothing. If you have the arm parts, you can go back to Tunnel Rhino's stage to get the things you missed there.
NEXT: Crush Crawfish