Give your Pokemon a soothebell to hold, feed it pokeblocks it likes (i think there is a man who can tell you what it likes in mossdeep city) and i think it would help to win battles lots with it.
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If you put Soothe Bell on a Pokemon or give it Pokeblocks.
There is no specific level but it only evolves when you have max friendship and are bonded.
At pacifidlog town,go down and go left.you will see a house, go inside.talk to the old man.
make it hold a soothe bell. make any pokemon hold the soothe bell and it'ss increase it's friendship
its not an emerald its RAYQUAZA the main legendary Pokemon in Pokemon emerald.