Since Megrez is just going to summon more, focus on weakening the mobs (I.e. killing the side demons, but leaving the leader alive in a compromised state), rather than outright destroying them. Doing so will save you resources for the boss while simultaneously draining the powers of any enemy healers.
Of course, with the mobs still alive to some degree, you're still likely to face the issue where you find yourself hopelessly cornered on the far side of dock. With the enemies weakened, I suppose you could fight your way back through, but then you would once again have to deal with the issue of regenerating mobs.
A smarter strategy would be to equip yourself with a genma or two, the reason being that the genma racial skill enables you to pass through obstacles, including enemy teams. The avian racial skill *might* also be useful for this purpose. An easy way to find out is to have Yamato on your team, as he comes equipped with an avian Suparna.
And of course, make sure that you go to battle with ice dances and media
