In a game I recently played, the question came up about the timing of Element of Surprise and Stealth Tactics.

I moved Obi-Wan Kenobi, who had been out of LOS, up to attack a Royal Guard. I play Element of Surprise, and my opponent plays Stealth Tactics. I can remove either of the dice, or can I only remove the black die, since the white die was not in the pool when I played Element of Surprise?

You can't remove the white die added by Stealth Tactics with Element of Surprise.

Element of Surprise trigger is declare target, step 1.

Stealth Tactics is while defending, so it needs to happen before dice are rolled, i.e. immediately before step 2. This is after declare target.

(And even if it were, defender abilities are resolved after attacker abilities).

Thanks, a1bert!