Fun concept and really lovely pixel art! Like some of the other commenters, I didn't figure out that I needed to match colors until I had died a few times. But after rereading the instructions I had fun. Trying to kill four slimes in 60 seconds gets pretty intense!
I wonder if it would be fun to add a bonus if you kill multiple slimes using the same set of cards. At first I thought you had to kill all slimes with the same set, but when I realized you got new cards after each kill, I started hitting the Cast button as soon as I found one pair of numbers that worked. It's much more challenging to get multiple kills from the same set.