Focus on the spikes. WASD can easily be dealt w/ peripheral vision, don't let red stray too far away, and just switch between space/no space for purple.
It's strange how there are none of those nice compliments we usually get in the top comments, but rather a bunch of smartassish, but honestly quite funny, remarks. :D
Man, you got the difficulty just right! Most TD games are too easy, or start off easy then jump to hard. This is perfect! Possible to beat, yet difficult. I also like how you made the spells' time reduced when you skip waves, most developers would have missed that.
I found an easy way for 39. First, fling yourself up to the ledge with the blue plasma by placing a portal under it and parallel to it. Then shoot portal A parallel to the turret. Portal B on ground. Cursor behind turret. Step through B quickly place B behind turret step through A. KO turret take cube to exit room. Go back to starting point portal A on wall above blue portal B on ground, through portal b onto, fling onto ledge. Portal A anywhere portal B on wall parallel to the one above blue. Step half-into the portal so you're sorta floating then jump and portal A behind top-left sentry. KO turret, take cube, go down, press button then portal a behind sentry, portal B on wall, through portal B, KO sentry, take both cubes into exit room so you have 3 cubes. Make sure they're all close to you. place 1 cube then jump on. Pick up another, jump then place below you. Repeat, so you're standing on a stack of 3 cubes then jump into the exit.
@MC Panda
Actually, what I did, is that I sorta got the evil dragon's jaw before mission 4 but I couldn't use it... However, I had tricksters with hellbreaker and I just sent them all out at the same time.
PS. Minecraft FTW