What u need:
-1 blood mage
-3-4 peasants
-lumber mill
-shop (to be able to buy clarity potions for mana)
-a map with forest behind enemy base or gold mine

First u go milita with 3-4 peasants and go with them and your bloodmage to the forest near the enemy base. Try to avoid dectection, and perform flame strike at a area in the forest( try to only burn one tree in the entrance to avoid suspicion) when you are at the end of the forest with your blood mage. leave one line of trees so u enemy wont detect u. After u build 3-4 or more towers at the burned area and wait for them to upgrade to guard towers. Then u burn down the last trees so your towers can attack their base u can repair your towers or building farms to block or your enemy can destroy your towers more easy. NOTE: works only on some maps like gold rush. Think of that before you do this