Take Phoenix the Charmander. Phoenix has always wanted to fly, but it angry that it doesn't have wings (yet). Therefore, it refuses to use STAB in protest. When it solos Lt. Surge to evolve to Charmeleon, it gets even angrier that it has evolved and STILL doesn't have wings. So it must learn Dragon Rage and use it twice in every Trainer battle it is involved in. After soloing Blaine and becoming Charizard, it is so proud of its new wings that it decides to use them to their fullest extent. Therefore, Phoenix must immediately learn Wing Attack, Aerial Ace, and Fly, the only three Flying-type moves it can learn in Gen 3. (The final move is up to you.) Finally, as a Charizard it must solo either Lorelei or Bruno.
Recap:
-Phoenix the Charmander
-As Charmander, no STAB allowed
-solo Lt. Surge to become Charmeleon
-must learn Dragon Rage as Charmeleon and use it twice in every trainer battle it appears in
-solo Blaine to become Charizard
-learn 3 Flying-type moves ASAP, solo Lorelei/Bruno