Mixtape mixing
Volume fluctuations? Good compression I suppose; automation if it's real bad. Or you can craft some of the terrible sounding stuff into the beat itself (automate drops or slap filter + fx automation over the whole track if it's a 2track). Creatively try and sculpt the weaknesses into the song would be my suggestion. See if the artists are willing to let you layer other sounds over it (transition sound fx etc), it's probably not even worth arguing over production credits (whatever small %) because a good mix will strengthen your reputation. Splitting a 2-track into different frequency bands, and treating them differently might be worth trying.