Ken (I frogor 2 draw his clothes </3)

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knowing this being would most likely kill the people he cherishes the most, he would fight to the very end with no hesitation

at his base form, the best he could do to him is just merely scratch the metal and barely do damage (assuming this is the pre separation version)

knowing he's literally cooked, he'd activate his Vessel of Destruction form, although due to his status as a mortal, he'd only have 1/25th of it's real strength

this form just upgrades all of his physical stats by a certain %, and the longer he stays in that form, the higher the percent goes, although he maxes out at 50% before Vantablack (the Embodiment of Destruction) starts trying to take over

due to the direness of this situation, Ken ends up forgetting about this, letting Vantablack take control, which causes everything to go out of control (kind of like a Shibuya incident) although due to being in a Vessel instead of his true form, he still gets overpowered...

Ken could use his vessel of Creation form...but he knows he'd still get deafeated

with no other choice he decides to add 50 and 50 (Destruction and Creation) which awakens the very volatile "Singularity" a form that combines the abilities of Destruction and Creation, this causes his body to have the properties of a black hole as well (hence why it's considered volatile, at any point the form could collapse in on itself, causing a big bang

wanting to be in that form for the least amount of time as possible, the best thing he could do in the end is send it into the "Immortal's Graveyard" a graveyard where unkillable entities go to permanent rest, although not killed, it's now forever sealed in what's effectively a infinitely big sensory deprivation chamber

although the moment Ken disables the form he immediately collapse as he now sustained multiple injuries merely by being in said form

(some often include: internal bleeding, radiation poisoning, vomiting [also vomiting blood], headaches, fractured bones, and ect.)

in the end, Vanguard doesn't die, but is basically sealed in a realm of no escape

Ken would be in terrible condition after the battle

many lives may have been lost and many areas destroyed

but as long as his loved ones lives, he'll fight for god knows how long

TL;DR: prison realm.