An Intellect Devourer gains the following benefit from its Body Thief ability:
While inside a creature, the intellect devourer has total cover against attacks and other effects originating outside its host.
The point of this is to find a viable solution to take out multiple Intellect Devourers quickly, from inside the host, while using as little consumable resources (such as Spell Slots or Items) as possible (or none).
The closest I have gotten, thus far, is trying to attack it with either Vicious Mockery or Sacred Flame. However, both
Vicious Mockery: You unleash a string of insults laced with subtle enchantments at a creature you can see within range. If the target can hear you (though it need not understand you), it must succeed on a Wisdom saving throw or take 1d4 psychic damage and have disadvantage on the next attack roll it makes before the end of its next turn.
Sacred Flame: Flame-like radiance descends on a creature that you can see within range. The target must succeed on a Dexterity saving throw or take 1d8 radiant damage. The target gains no benefit from cover for this saving throw.
both specify a necessity for you to be able to see the target. The quickest, easiest (but unviable..) "solution" I have, so far, is a Celestial Warlock with Pact of the Chain using a Bat or Pseudodragon familiar, choosing to see through its senses for its Blindsight, and then casting Sacred Flame at it.
This method is thwarted by the simple fact that (both via Twitter), while Jeremy Crawford has confirmed that blindsight qualifies for anything in the D&D rules that requires you to see something (as long as its within the radius), he has also confirmed that blindsight does not beat total cover. Meaning a working solution may be somehow sharing senses with a creature that has Tremorsense. Casting Beast Sense comes to mind, but I am unaware of any Beast-type creatures with Tremorsense.
Truesight does not detect an Intellect Devourer either.