Thanks for the question -- an excuse to return to Vim Adventures for the first time in a while! Thanks!!
I've just confirmed that it's possible to edit that text in the required keystrokes, with the keys you have now:
My keys collection matches yours early in Level 6, and I edited the 'asdf ' word with strokes to spare, releasing a yellow key.
Without giving too much away -- I didn't do it on the first pass through; and it really helps to approach it backwards.
I hope that helps; and if you're still stuck, I'll give a more complete answer.
Happy Vimming!