This will be kind of a grind as you will get all the other endless mode achievements before this.
Just start up an endless mode and just keep making 3 letter words. You may kill a few monsters along the way, but you won't make it very far. You wanna create as many words as you can. Just rinse and repeat.
By the time I had all of the other endless mode achievements, my progression was around 40%. So this may take a few hours of grinding at best.
I hope this helps. Happy hunting!
Edit: estimated time after getting the 2500 letter achievement (48% progressed) using mostly 3-letter words was around 1 hour. So start to finish may be ~2 hours.