Best way to defeat the elite 4 on Pokemon emerald?

Basically, any team with decent type coverage will be fine, as long as they're all at least Level 50 - over 55 might be a good idea, and if not you can go back and wander around the exit to Victory Road and train. Rayquaza is a big help here, so if you don't have him, I recommend going to Sky Pillar to the east of Pacifidlog Town so you can catch him first. He's level 70, and a HUGE help.

No special movesets are needed for beating the Elite Four, so TMs aren't really a necessity, just give them some good moves from levelling up. Make sure you buy PLENTY of Full Restores and Revives before you challenge the E4! Even with an amazing team, with no recovery items you'll never make it.

So you can train your favourite Pokemon rather than the strongest, and they'll be fine provided the movesets, levels and items are decent. My team was Sceptile, Flygon, Camerupt, Seadra, Magneton and Aggron. Not the strongest in the game, but they sufficed.