No unevolving Magikarp? You read my post.
Anyway, take a Growlithe. It must learn Dig and Iron Tail and never forget them.
In order to evolve, it must solo Erika, and all the trainers in Pokémon Tower (except your Rival).
Finally, it must solo Lorelei's Lapras, Jynx, and Slowbro to continue.
It must be Male.