Suggestion: you should make a new monster that has the ability to attack fast. like zombie horde makes 100 DPS, you should make this new monster (let's call it BinuM's pet pls b0ss) makes 50 dmg but like in half of second or 0,1 or 0,2 of a second. It won't give op easy finishing the game, but it will be easy to clear more houses in less time that don't have too much HP.
Building has 50 HP, monsters do 100 dmg, binum's pet does 50 dmg, monsters 1 building pro second, binum's pet 2 buildings pro second.
BinuM's pet > monsters. late game monsters > binum's pet. Thank you.
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