At lvl 1, my genesis did ~15k (13-18k or so) on elementally neutral monsters, and up to 28k on holy-weak. I currently have genesis at lvl 5, it does up to 21k on elementally neutral and I've seen it break 30k at himes. Getting maple warrior to 9 before I dump anymore points in it. I know I'll eventually need it at 10 to 2hko skelegons at a reasonable level, but for now, this is letting me only need two gens on himes and then a few SRs/Bahamut attacks to finish 'em off.
Also, only 18k on elementally weak? Are you sure? Keep in mind most of the 'dark' enemies around leafre aren't actually holy weak.