I got the achievement yesterday without cheesing...I tried it a few times at first, but then I found out the wizard towers in that top corner were still too much of a pain in the ass to keep in check. So I just did it the 'normal' way. Fly around like a madman, burning princess coaches left and right.
There are a few spots on the map (particularly left and right sides) that aren't covered by more than 1-2 mages towers, so if you can keep them in check a bit you can fly around between those areas to keep the knights busy, and torching coaches. I'd usually pass behind the castle/city in the centre of the bottom of the map, since only the archers can get to you there and they're not fast enough if you fly in a straight line.
Apart from that, I checked out the game a bit more, and I'm sorry to see that the rest of the game is just...breaking your own high score or getting "gold medals".
I mean, it's still fun, but I'd rather have an actual challenge or goal or a narrative. Some sort of campaign or something. It's a shame because it wouldn't be hard to put in diverse goals per mission.