preventing air raids?

[Braininthejar]

Now it's their turn to watch out for surprise attacks. 

Yeah, and the key thing here is that it's not just you being a mean DM.  There's a perfectly reasonable, in-story, reason for some magical sneak attacking.

Of course there is. He had been looking for them since the day before, when they raided his coven's secret base (basically they decided that an open war against the most powerful wizard coven in the country will be a shortcut through the plot, without checking the enemy strength first.)

Fortunately, the guy usually arrives via teleport, so the team wizard's anticipate spells gives them enough warning to port out before he ports in. 

[dither]

Wizards are really smart and tend to have magic and/or money.
- Raise/hire a small army of minions -- undead, mercenaries, or followers.
- March the small army into the swamp, armed with simple bows or crossbows.
- Disguise self and hide among the minions or follow with invisibility (suggested above).

Surprise rounds become a moot point.

If the druid(s) are foolish enough to give themselves away by attacking, roll a LOT of Spot/Listen checks.

Have the low-level minions tear apart the swamp looking for the PCs. Maybe the 7th-level city guards aren't an option, but the wizards could charm person an entire 500-person village and bring them along as "hostages." Then watch the PCs rain death on that.

edit: Oh, I was ninja'd forever ago. I skimmed to see if these suggestions were here and only added 'cause I didn't see them.

I think they bear repeating however.

--Dither