Shapeshifter hollow III

Hello everyone, 

I’m having a problem with the level “Shapshifter level 3”. The problem is that I’m not able to finish this dungeon (99% completed); because I’m not able to completely destroy the  firebolt tower located on the right of the dungeon’s skull (red arrow on the image)  . I’m actually destroying the dungeon much quicker than the firebolt tower, and the fact that I have Kaiser as a pal is not helping either. 

Is there any trick to finish this dungeon, or destroy the firebolt tower from afar ? I’ve tried 6-7 times, and I’m blocked at 99%, it’s quite frustrating.

Thanks

Select cannons, they will destroy it from the right (half-way)

 

Maybe insta-cannons

This is my version of the dungeon. My troops and spells are slightly higher levels than they were when I initially played the dungeon for the first time