I found a relatively quick grinding method. This was my very last missing cheevo.
1) Increase the health of your character as high as you can.
2) Equip this fully upgraded book: Heel Me (2nd book in the list, the two other books are irrelevant in my grind.)
3) Equip fully upgraded Light-Scythe (8th in the list) which reduces receiving damage.
4) Choose Stage 7, where the giant rabbit takes no damage from 3 letter words.
5) Just hit him repeteadly with valid 3 letter words no harming him at all! This is the idea. You die only after 20-35 or more successful words, retry until you reach your goal.
Words containing "E" heal you.
For me 500 words took about 1 hour or so (adding: English is not my native language).
MarekVit's addition: The achievement tracker progresses even in endless mode.