Garanteed win :
Get rid of all the green cards. Only keep red and blue, 5 schools, 5 sorcerers, and plenty of cards to build massive things. You can keep 1 or 2 banshees if you like, but most of the time, a small deck full of red/blue is enough to build up to 100+ in no time.
There is one big bug, if you shoot the last bullet, the one that completes the level, and then you die WHILE it's flying, you get the "you lost" screen but right after, it's an automatic win with 0 secs and 0 shots taken.