I would definitely need help sorting out the details, but I was thinking something along these lines:
Goblin-based (since they're sneaky & quick.)
Can ignore (jump over) walls
Runs to the nearest defensive building and starts "attacking" it. Its attack doesn't do actual damage, but when the "damage" done is equal to the building's total HP, the Saboteur(s) takes over the building instead, and it will start attacking its own CC troops, Heroes, or other Defenses. Once all CC troops, Heroes and other defenses in range are destroyed, it would then switch to regular buildings. I've tossed around whether or not those formerly friendly troops & defenses will attack the taken-over one. I think at the very least any troop/building that it actually attacks will fight back.
Once the Saboteur is engaged with a defense, that defense can no longer see the Saboteur, so will ignore it. If it wouldn't be too big of a programming issue, possibly have any other defenses ignore the Saboteur if they don't have a direct line of sight (the Saboteur is hiding behind the structure it's trying to take over.)
I'll admit that I think it's quite likely this troop will be denied, due to complexity issues, but trying to fix that would be part of the needed help with details.
