it's really fun! and with a little tweaking and balancing i'm sure you can end up with a really good game
i played on easy and it's way, way too hard (my best score after 15min of playing was 18 :( ) - two things comes in mind: the amount of enemies and the fact that they tend to stack (so if you teleport to one enemy you'll end up being hit)
my two cents: you could have the amount of enemies increase crescendo, as well as in waves so the player have time to breath between moments of intensity (you have an amazingly intense game) - perhaps you could start with only one letter and add letters little by little to increase the difficulty