Do I have to level up each hero in the Bond to increase the bonus? For example, I have Minos, Bragi, and Merlin in the Priest league which starts at +300 MATK bonus, do I have to level up all three to get the highest bonus?
It mostly depends on exp gain. Awaken grants 5 exp while Promotion gives 20. So just focus on the Mercenary which you use and once you get max exp from that mercenary, choose the rest to make the lvl5 bond.