+1 Goblin upgrade is great for stage 4. At the start, put a goblin beside one of the guarding goblins. When your goblins have populated, kill the enemy goblins and dig out the treasure while your hero sucks up centaur arrows. Getting the treasures makes stage 4 much easier.
Manticores are great, since they are essentially a very long ranged attacker, super magic resistant, and are buffed by the Gym, just like all your tanking creatures. They make later stages easier.
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