google "adult seo", the ones ranking are probably the ones who know what they're doing
tbh tho I've had zero success (and spent mid $xx,xxx) hiring "experts". They all seem to be stuck in 2006, focusing on keyword/anchor analysis and non-contextual header/sidebar/footer links when in 2018 you need brand recognition, co-citation/co-occurrence, and contextual links. Churn and burn with cheap/spammy link packages from blackhatworld, however...that shit actually works for me, just gotta make new sites all the time.