Undyne simply glared and looked up, Throwing her energy spears at where she assumed the Bombs came from, before running off with Daisy

"Oh no! He's here! Ahhh!"

Ace swooped down and snatched Destra, "Nyeh heh heh!" "Er... Uahahahahaaa...."

Undyne began baraging Ace with her spears

He gripped the ones close to him and threw them back at her. "Well isn't this a fun little game?"

Undyne was clearly unimpressed "How about we do this as a nice one on one?" Undyne suggested "Me against you, If you win, You do whatever with those two, If I win, You just go off and die someplace." Undyne said smiling overconfidently, still throwing her spears (And By throwing, I mean launching 3 at once. Multiple times.)

"What if I decline and play dirty like always. I don't follow the rules..." He said, laughing. "You're nothing but a nuisance."

Undyne simply smiled, continuing to throw her spears

He laughed, growing an utterly disturbing third eye on his forehead. He began barraging bombs, dark beams, and pure shots of hellfire.

Undyne smiled, "Is that all you got?" She said cockily
a damage meter appeared over Undyne's head
-1281

She did her BS circle spear attack on Ace

(That's easy to avoid.)
He evaded, then his arms began molding together to make a flesh cannon, shooting dark matter.

(It isn't.)
-1236

Undyne did her small 3 spear attack, and suckerpunched Ace at one point with one of the spear

(Trust me, there's a pattern that works.)
Ace was tossed a little, but began shooting his cannon, which was oozing blood and other body fluids.
(That's the second most disgusting transformation he has.)

-2000

Undyne smiled using the attack again

"How can you expect to defeat me if you are beaten by a child?" He says, shooting more dark matter at her.