might i suggest making the enemies strong/ weak versus different types of fire so you can't get by with just machine guns but you have to build shotguns/snipers as well
great game, my only complaint is that on the mode where the shapes randomly pop up on screen, occasionally red shapes appear on top of my circle and i lose my multiplier, other than that amazing game 5/5
MDG401, the lord of hell absorbs most damage, i would suggest using only dragons, titans, and 1-2 angels to take care of units that spawn behind him (angel's attacks heal him but only slightly compared to the damage he takes from titans and dragons)
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