Japanese Diamond/Pearl
Mimic Glitch
-Your party needs a Ditto, Pokémon with Mimic (E/FR/LG has Mimic Move Tutor) and Pokémon with wanted moveset (I suggest getting Smeargle)
-Go to 2vs2 battle
-Let your Ditto use Transform
-After transforming, be sure your Mimicker is faster than Ditto and target your Mimic to Transformed Ditto
-Mimic copies the Transform
-Target Mimicked Transform into your Ditto
-On Dittos turn, switch Ditto to Pokémon which moveset do you want to get (Smeargle)
-After Trasforming, kill your Mimicking Pokémon
-Bring any Pokémon on front and win the match
-Now your Mimic Pokémon has forged all its previous moves and knows the moves of the Pokémon which you Transformed into.