Best ways to kill a Tier 1 caster without being one

[Emperor Tippy]

If he's always hiding on his private demiplane I don't really know why you'd need to kill him.

Astral Projection. A Simulacrum of yourself plus a Permanent Telepathic Bond with it.

A wizard can interact without any problem, even when remaining in a secure location.

For day to day stuff and other activities that aren't likely to gain you experience; Simulacrum's are your best bet. Sure they are half your level, but that is still 10th level at level 20. More than enough to handle day to day affairs at court or in a city. When something more is needed your Astral Projection can arrive quickly and easily.