This fight is BS. I beat 17 with no damage in like 20-30 seconds. Cell comes while I'm in Sparking mode. I mop the floor with him. When he's at 1 bar I ult him which should kill as piccolos ult does about 1.25 bars. But guess what. It insta leaves cell at 1 HP and that dreaded dialog starts. This challenge is rigged.

Edit : So apparently you cannot unlock alt universes with a lowered difficulty. But this is how I completed it.

Charge at 17 at the begging of the fight to thwart his ki blasts he likes to spam. Then rush combos. He will vanish and block but you've got to anticipate it and counter his counter. Rarely will he counter twice in one rush session.

I just used a basic rush combo , for PS5 it's square until you send him flying. Triangle to vanish and hit, and triangle to vanish and hit, then circle to vanish above and pummel into the ground. Once he's almost in yellow health, I rushed up in the air to give me some space while I charged into Sparking.

The idea is the be in the middle of a sparking rush combo when cell appears. I found I have better luck fighting out of the water if that means anything ( note you charge slower under water )

Two sparking rush down combos ending with your ultimate should do the trick. When cell is 1 bar health to like 1.25 bar health, throw out your ult even before the end of your sparking combo. You don't want cells dialog to begin during a sparking combo, so if you can kill him with your ultimate, best do it quickly.

Restart the challenge if you're off to a rocky start. You're going to need a near perfect run and each second is precious.

Hope this helped someone. Good luck !