doctorbadwolf
Heretic of The Seventh Circle
I’d treat it as a social interaction challenge. Since the ettin is almost certainly hostile to the PCs, it being in combat with them, to successfully provoke it at all would require a taunt which touches upon some ideal, bond, or flaw it has. To discover what is going to get under its skin requires knowing how to speak Giant or Orc, and using an improvised action to interact with the ettin enough to make a Wisdom (Insight) check. (I think I’d set the DC at 9 due to the ettin’s low Charisma.) Success on the check would reveal a personal characteristic of the ettin which could then be used to taunt it (on a subsequent turn) by making a DC 20 Charisma (Intimidation) check. If successful, the ettin uses its next action to attack the speaker with advantage.
edit: This is why I voted for the second option.
That seems to me like a stealth “no”. Why not just say “no”, instead?
Taking an action in combat that has some genuine tactical payoff potential should generally, if successful, provide an actual benefit. Two actions just to give an enemy advantage against you is...the same as saying no, but with a punitive excercise in teaching the player not to ask.