Palskane said:
But as I said, they are going to rescue another PC who has been captured. I really don't want to kill them.
I understand that. When I GM, I don't
want to kill the PCs. Unfortunately, due to a variety of circumstances, it has happened before.
The PCs' best defense against this sort of foe is a good offense. They need to gather intelligence and strike by surprise after working out a coordinated plan of attack. If they know what the parameters are, achieve surprise, and act as a team, the foe isn't going to present much of a challenge.
If, OTOH, the players refuse to exploit their PCs' capabilities (and the group you describe ought to be quite capable), then let the dead bodies fall where they may.
Now, of course, if the PCs are supposed to win, then there are ways to stack the deck in their favor on your end of things. For example, perhaps the enemy has been sick lately. Apply some ability to damage to STR, CON, or whatever. Maybe the enemy is too overconfident, and therefore doesn't fight effectively for the first few rounds. Et cetera.