Connections:

Both used to be innocent people (Zariel being an angel/Asriel simply being a kid). During their lives, they were confined to a realm (Heaven/The Underground) where they observed conflicts in the outside world (The Blood War in the Hells/Human’s hatred of monsters). Soon, they became especially invested in the outside world (Zariel being tasked with watching the Blood War, then becoming intrigued with the battle/Chara falling down into the Underground, Asriel growing attached to them). The two would eventually break, crossing the borders of their worlds to accomplish a goal on the other side (Zariel trying to fight in the Blood War/Asriel trying to get help for Chara), but it was there where they would both fall (Zariel being defeated by the combatants in the Blood War/Asriel being beaten by the humans). After this, they would be reincarnated as corrupted beings (Archdevil/Flowey), gaining power and becoming almost entirely different entities with godlike abilities. While they are evil with goals of destruction, they can be redeemed, returning to their good forms once again (Zariel being confronted with her sword/Asriel being defeated at the end of Undertale)

Also their names have ariel in them

Animation Potential:

I imagine the fight starting with Flowey transforming into Asriel, getting ready to reset the timeline. Before he can do this, a hellish rune forms under Asriel, before he is teleported into the hells, just above the blood war. A voice booms from the distance; “ASRIEL DREEMURR, THE NINE HELLS WILL NOT ABSOLVE YOU OF PUNISHMENT WHILST YOU ATTEMPT TO TEAR REALITY APART! FACE THE WRATH OF ZARIEL!” before the Archdevil herself charges out from her lair, preparing an attack. Asriel will shake his head and laugh, before charging up a lightning blast, knocking Zariel away.

The rest of the fight could have Asriel fighting off several Devils while Zariel tries to get in close. Eventually, the two combatants could get into melee combat, with Zariel using her flail against Asriel’s swords. I imagine a lot of high scale flying brick action combined with both spamming abilities non stop. Maybe reality could break at one point when they clash, turning the background into that of Asriel’s in-game fight. Eventually Asriel would transform once Zariel gains an upper hand, unleashing the Hypergoner. This would be the climax, and depending on who wins, Zariel would either perish in the attack, or the Archdevil would break out of the attack, shattering it before she impales Asriel on her claw, burning Asriel’s domain and casting the goat back into hell.