whatever i do i can't beat demon 1v1
Just like in normal maze TDs. Making long mazes is usually bait (unless you're abusing some kind of money over time strat). You make one just long enough that your "kill zone" towers have mazing inside their full range. Any mazing beyond that doesn't actually do anything but waste money. Then dump all money into making that "kill zone" as strong as possible. By upgrading the units inside it (Damage>Armor>Health>>Aura). 2-3 of each type inside a single aura towers range will outscale the AI. About 2½ lanes of towers is enough for almost full kill zone saturation.
I beat it using the literal worst possible unit, that's countered by demon armor, doing that.
Doing the same thing also works in low ranked Multiplayer, but once you start ranking up. It just gets outscaled. The Gold tower is just absolutely insane. You basically make a full saturation maze of Scaffolding on Day 1. Put one of each tower (except Chaos) in the middle of it. Then level up the gold tower and it will give you like 1k gold per day and you just snowball and get a single Fire Mage to stack all your +Power on and it just oneshots the whole battlefield at 0s.