The wording of "Stunned" seems inconclusive to me. Does it ignore incapacitation immunity? If not, do other effects apply? Specific situation: what happens if I successfully hit an Air Elemental with monk stunning strike?
In your specific case with an air elemental:
MM, Page 124
Condition Immunities exhaustion, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious
There is no condition immunity for Stunned or Incapacitated so an Air Elemental would suffer from all of the Stunned conditions (included incapacitated).
I tried to search the MM and could not find any Monster that had the condition immunity to Incapacitated.
A stunned creature is incapacitated (see the condition), can't move, and can speak only falteringly.
The creature automatically fails Strength and Dexterity saving throws.
Attack rolls against the creature have advantage.
Stunned includes Incapacitated, but RAW doesn't say it ignores Incapacitated immunity (so it doesn't), and Incapacitated immunity doesn't mean Stunned immunity (so other effects apply).
Perhaps if you gave more information on your specific case it would be easier to understand why you are in doubt.
\$\begingroup\$ Nothing to elaborate, really. I want to stun a creature with incapacitation immunity, I.E an Air Elemental with a monk stunning strike \$\endgroup\$– MisamotoMay 4, 2016 at 19:04
\$\begingroup\$ @Misamoto I don't have the MM on me, but if it's immune to being incapacitated (which is not being able to take actions), it will take actions even stunned, but it can still be stunned (ie. suffer other effects like fail STR and DEX saves and advantage against it). \$\endgroup\$ May 4, 2016 at 19:11
10\$\begingroup\$ Air elementals are immune to several conditions but incapacitated isn't one of them. If it were you would apply as much of the stunned condition as you could, so everything but incapacitated - can't move faltering speech, failed saves, etc. \$\endgroup\$– yinzanatMay 4, 2016 at 19:13