I want to build an encounter with an interesting hook - the opponent is hard to defeat with anything other than AoOs. That's why I'm looking for ways to somehow deny the PC (it's a solo game) his normal attacks or make the enemy hard to hit/invulnerable. Maybe something like very high AC which is up only on your opponents turn?
An important houserule of mine - enemy is flat-footed when you make an AoO against him.
I assume that #1 suggestion will be a high Dex enemy with high dodge bonuses, because they're not added when flat-footed.
The enemy is supposed to fight melee, but spells for buffing or defense are allowed.
He's going up against a 20th level PC.
