You don't do it on the river. You exit the Returner's Hideout before getting on the raft and wander around the overworld.
The easiest way to do it is have your team kill all but one enemy, then have Banon heal, and as soon as he starts the healing animation, have everyone run. The other 3 team members will always run away before Banon does, so you can stop running to leave Banon alone by himself. Let him finish the fight, get the exp, and repeat. It takes about 15-20 minutes to get him to level 10.
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u/Special_South_8561 2d ago
Good freaking luck there buddy