Big problem I had when things started getting hectic (120+) is that sometimes I'd go to type a word—let's say "frack"—and it wouldn't register me typing that word. Why? Because "fudge" (another word that starts with 'F') was somewhere else on the screen, thus ending my streak and making me increasingly frustrated. It feels like a coin flip at that point as to whether or not it picks the word I want, and after losing too many in a row it cascades. At one point I was desperate to get 2 more swords so I could clear the screen, and one dropped on the ground. It had the word "raspberries". I went to type it... turns out an enemy had the exact same word, and it prioritized them over the sword.
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.