Invest some of your skill points into heal and stack mana regen on your armor. You'll be invincible. Don't worry too much about individual element defense status until floor 40+, that's when elemental damage starts to pick up starting with the wizards. Bleed gets the most bang for buck.
I like the idea. It's unique, but it could be done better. Movement feels a bit slow with the hold down to move type system. This in turn makes the gameplay feel slow. Maybe you could make limit the amount of taps possible in a short time via a cooldown. This keeps your balance in the game. Then also increase give an increase to speed. Holding down for power should accelerate quickly then slowly.
Difficulty needs to be tweaked a bit. Spawning melee guys, I can handle those. Spawning a wave of guys with guns, that's like 5x harder. Always gets me killed as well.
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 "Demons Down Under"){kind=icon}
 "Bullet Heaven 2"){kind=icon}
 "Elephant Quest"){kind=icon}
 "Dodge The Asteroids"){kind=icon}
 "Get Tested!"){kind=icon}
 "Bob and Weave"){kind=icon}
 "Magnetized"){kind=icon}
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 "Gnok"){kind=icon}
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Most of the games I design are made that way. I'm an 80s kid- I like retro stuff.