TIP #2: If u want a full scale charge let ur hero go foward in the beginning and let ur units stay... but when ur hero reaches the half way line send him back near the castle and keep on doing this routine so ur units can build up then when ur units is fully spawned begin ur charge of VICOTRY!
Tip: When the enemy revenge wave is coming and ur units ar dying, let ur zombies go foward while u retreat ur other units so they can regen hp or respawn, ur zombies will give u time :D hope that helped!