It's hard to say if a VimGolf challenge is "hard" or "easy". Every challenge is easy if you waste enough strokes. In Reverse Simple Deletion, just one stroke separates an easy 12 from an unreasonably difficult 11.
Prime Numbers is among the hardest challenges in VimGolf. I've spent more time working on this challenge than any other. When I first saw it, I figured I'd have to brush up on my vimscript. As it turns out, you don't really need any at all.