I learned decades ago, if the party ever sees the enemy, they can kill it, no matter who or what the enemy is.
Heed Slim Jim's warning. No metagaming. Have you considered illusions? If you are creative (i.e. get out of the rulebook and start thinking), the characters will not know which way is up. Meanwhile, the enemy tests their responses and returns prepared to deal with these wannabe rescuers. The players will realize that the opponent has optimized against them because they showed him all their tactics.
Meirril said: "To be honest, there isn't anything wrong with having a player around with really good AC. That doesn't make them invincible, it does make them unattractive as an opponent."
Correct. My 4th (now 5th) level monk has a 29 AC. Against a rival faction of monks, the mooks got trashed so the leaders fought smarter. Now they run get bigger guns when they see him coming.
Thunderlord said: "I'm pretty sure grappling a tank is effective."
Do you want to rethink that? Where does this guys AC come from? What do you think his CMD will be?
Last thought. What kind of an evil mastermind would leave the princess in need (or, we are equal opportunity here, the hag paladin) in a location known to the rescuers?
