[Arkcon]

A simple trick is to make a small fleet with specific orders to target smaller, non-weapon ships. These ships tend to carry supplies, anti cloak sensors, mine sweepers, troops, etc. The loss of the ships specifically will slow down the advance of a fleet.

Also, try diplomacy. Tell his allies that he's too big and dangerous to everyone. Try to get a few turns where he has no allies, so he'll scatter his fleets. His friends can come back to his side once the battle fronts are scattered.