Hello and thanks for playing my game! I've just uploaded a new version of the game that fixes the above bug, as well as introduces some balance changes that should make the game much more fun! Please give it another try!
Later stages are much better now! I see you don't spawn anything with the same letter, however the monster pen can still have a word that starts with the same letter. After your changes I got 825 kills, 59 treasure, and 37 max streak. After a certain point in the game, you start only killing enemies to get the sword drops, and then clear the screen. You might want to lower the rate of sword drops, because that's what carried me past 300 kills.