Or, like I said, fighters.
Again, I have no experience with Devnull, but if fighters still have their intrinsic to-hit bonuses (both offence
and defence) they're very handy, especially because they're so cheap. Depending on the kind of guns he's slinging, this might also give you the advantage of rendering a certain amount of the weapons on his ships useless.
Or, if there are more players in the game, seek contact with other empires and request that they attack your enemy in return for trade goods (resources, outdated ships/units, defense bases which have become useless, listening post bases which have become useless due to your exploration, planets with low value and no population as a result of, say a plague, etc.).
Or, building on the above paragraph, build up a powerful static defense line of bases, sats and especially mines, then find another (preferably weaker) empire and simply outproduce them, then bully them into attacking your enemy. If he doesn't listen take/glass a few unimportant planets and post traffic regulators on the warp points to avoid his ships colliding with each other on their way to your enemy
