Raksuma

You fight it in West Cave Floor 2.

Its Nexomon's levels are rolled when the battle starts, from the story's difficulty plus this battle's own weight of 3, as the Battles page explains.

It's a wild battle, but it doesn't allow capturing, so the capture option never shows.

Winning it gives ×2 EXP and ×2 coins.

What its boss cores do:

• A boss vitality core (100): 100% more max HP.