Shadow Psyduck

Water Gun + Aqua Tail and Cross Chop or Psybeam

Water Gun takes advantage of STAB, and performs much better than Zen Headbutt. 

Aqua Tail and Cross Chop have identical stats, though the former does benefit from STAB. Psybeam is generally a poor move that also requires a lot of energy, though it could provide niche uses if Psychic-type damage is needed.