Still a nice, fun game. I love that I'm able to go back to my childhood games and... Actually beat them for once, or at least get further than I did before.
The boxer at mission 4 is kind of OP, but I realized that you only have to survive for a few minutes. I recommend using weak units for mission 4.
If you want to defeat the boxer, I had 2 star mana regen. I place a couple of decent units to destroy the enemy spawner. Then, after the units died to a single shot by the boxer, I spammed 8 archers and did arrow rain spam. The archers of course died, so I don't think you need archers. You could just do arrow rain spam.