Boss monsters wanting to stay on this plane

[maniacalmojo]

So cthulu in power armor appears to wreck havic on the world for ****s and giggles when a wizard happens to walk by and cast banishment thus sending him back to his plane.

If a boss monster like that wanted to force themselves to stay on the material plane how would they go about doing it.

[Muggins]

Casting Banishment on Cthulhu would likely result in him occupying both the prime material and the plane he was banished to. Congratulations on making the problem worse!

[gooddragon1]

So cthulu in power armor appears to wreck havic on the world for ****s and giggles when a wizard happens to walk by and cast banishment thus sending him back to his plane.

If a boss monster like that wanted to force themselves to stay on the material plane how would they go about doing it.

Naturalized Denizen (http://www.d20srd.org/srd/variant/buildingCharacters/spelltouchedFeats.htm#naturalizedDenizen)

Naturalized Denizen [spelltouched]
You are unusually anchored to your location.

Prerequisite
Exposure to dimensional anchor spell.

Benefit
You are never treated as an extraplanar creature (and you lose the extraplanar subtype). Thus, you can't be affected by a banishment or dismissal spell or similar effects that send extraplanar creatures back to their home planes.

Dimensional Anchor (http://www.d20srd.org/srd/spells/dimensionalAnchor.htm) also works if they don't dispel it. Dimension Lock (http://www.d20srd.org/srd/spells/dimensionalLock.htm) does this for an area.

The simplest way is of course spell resistance because both banishment and dismissal allow spell resistance. However, it is not guaranteed.

Note that such spells are save or lose and that a player may just use a save or die or different save or lose instead.