The best place i found for this was the Tremel Tree @ lvl 16ish.
In this dungeon at the very start 4 groups of 4 cross paths. If you pull them together in a group of 8 they are very easy to kill and award 3k xp. It took me around 7 fights to get a 5 star bond.
Bond 1-3 are 1 fight each, Bond 4-5 are 2-3 fights each.
Very fast and easy with AOE damage.