So I'm playing Emerald and I wanted a good moveset before going into the E4. My Swampert currently knows:
-Strength (This move I want to change)
You can change the other moves if you like.
For the elite 4, your best option is Brick break. Brick Break is a fighting move that allows you to beat both Sidney's dark types and Glacia's ice types which means the only elite 4 member swampert won't be able to beat is Phoebe.