you should make a special lvl for xp after you finish lvl 12 or make normal lvl to give X more xp, make an option for hero when it have less than 35% hp to autouse first aid (hero use mana, if no mana hero can't use spell)
ubgrade your shild and altar at max
13-use it until you destroy the castle and skip black imps(go with full mana)
14-use shild when bomb explode, you also can use berserk to destroy it faster
15-use shild and berserk until you defeat that red guy
16-go with 200-300 mana and use shild until the enemy castle is destroyed
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