Actually, it depends on how many people are fighting in the battle. What I've learnt is that to keep getting experiance, get them at a much higher link or power then the opponent, then challenge them. If it's 1 vs 5/6, then you easily gain 5-6%. It drops down to a min of 1.5% if you have 6. I battle with my hero and nobunga, and they gain 4-5%.