Hey Stalin the best way to deal whit heavily entrenched enemy's is to use Parrot canons or Armstrong canons whit exploding shells. Whit those you can basically just bombard the enemy and their gates until you run out of ammunition and the move in for the kill.
Hey in the Campgain Map select 2 units that re hurt and do Ctril+M and they will merge and may gain/lose some expiriance its easier instead of recruiting units or waiting for them to regeneate so u can send 1 unit instead of 2 to a town for regen