Monsters that can phase should be able to escape bear traps without resistance. If they are solid like xorns, they should still take damage, but they should logically be able to just phase out of the trap.

Artifact chaotic helm Helm of the Underworld, modeled after Hades’ helm. Grants drain resistance, invisibility, magic resistance, and passive detection of buried objects (or phasing, if this is too difficult to implement) when worn. When invoked, it does a stronger version of the chaotic turn undead effect (rebuke or tame undead).

Artifact cloak of invisibility Death’s Embrace, which grants phasing, immunity to death effects, and half physical damage, but reduces speed.

All monsters of class W (except perhaps Nazgul) get the PASSWALL monster flag, allowing them to phase.

Wood nymphs, or players polymorphed into wood nymphs, can walk through trees as if they weren’t there.