The way undermine works is you click on a spot on the map. Undermine will then move from that spot to the closest tower/fort/TC and then continue on a straight line for the length of about 3 forts.
So NEVER click on a tower to undermine it (unless there is just the one tower). Click near the tower on the opposite side of another target.
I actually used the editor to make a field of towers. With a little practice you can take out a whole line of them instead of just one!