I disagree with the comment on not having level 4 letters. Level 5 letters like Q and Z are hard, but level 4 (W, Y) are critical to destroying the big bugs. Just memorize a few words to use: wavering, waylaid, weaponry, westerly :)
Misspellings will kill you at later levels. Just populate your boiler with longer words you can make. I find myself using "understand" and "overwhelm" a lot :)
Great game. People who are complaining about typing fast are missing the point. The key to this game is using words with rare letters--one W is worth 4 E's