Due to him able to change his physical form into pretty much everything...He is pretty much immune to physical damage
For example:Ink changes his body into...Ink
Ink is something you cannot physically destroy. This ability makes him intangible,
If that is the case though he might be able to regenerate from thin air.
You can permanently kill him but you probably need to destroy a few concepts to do so.
For example the concept of existence, life and death.
If you count this as killing him permanently you could theoretically erase him from all of reality and send him to the void forever.
Im not certain but yeah
Erase every atom of his being, every electron.
Matter can not exist without its bases
What do you think?