Glacier Knight, take a Sewaddle. You have two choices for how your Sewaddle can grown and evolve...
1) Name it Erika. This Sewaddle idolizes Erika and embraces its grass typing. It can only learn moves that Erikas Pokemon can learn (her Pokemon from RBY I mean, no cheating with HGSS, GSC). This includes the evolved forms of her Pokemon, so it can learn moves that Tangrowth could potentially learn.
2) Name it Bugsy. This Sewaddle idolizes Bugsy and embraces its bug typing. It can only learn moves that Bugsy uses can learn (meaning Scyther, Caterpie line and Beedrill line).
Either way, it must defeat at least 6 Pokemon collectively from the elite 4, N, Alder and Ghetsis. Good luck!