QUOTE(Comoop @ Nov 1 2017, 12:54)

Specially on the role of ん...
Some people do this in informal speech: r sounds (e.g. る or れ) followed by n sounds (e.g. の or な) might have the r sound substituted with ん because that's the sound you get when you say it quickly. e.g. するな said quickly becomes すんな.
In your case, くれない becomes くんない for this reason.
So you get イカせてくんないヘタクソ = イカせてくれないヘタクソ = someone so bad (at sex) that they don't make the speaker cum.
I'm not sure how that relates to the previous line, though, since that part doesn't make much sense to me.