[Fantomen]

If you move from a high dom province to a low dom province you don't lose the HP bonus right away. If the manticore stays in low dominions a few turns it should have lower HP.

With those paths I guess this is the AI you're facing, so it won't kit the manticore properly. Since you managed to get it to 99 fatigue you should be able to work that angle, and add in some better damage dealers to kill it.