[Alneyan]

All else being equal, your odds to hit anything at range 8 are of -5% (rounded to 1%), before the Stealth and Scattering Armours fail. If he is not using Baseships, you will hit only once every hundred shots, which is not that great.

But if he is not using Baseships, you should do so yourself; Repulser Beams only work if the attacking vessel is as big or bigger as the target, and so having a bigger hull than he does would make these weapons worthless. If he does use Baseships, staying at maximum range could work (because of the 40% penalty to defence of these baseships), but make sure your fleets and ships are trained all the way to 20% each.

Fighters, drones and the like would also work, since he is probably lacking heavily in PDC and other similar weaponry. Also make sure to fight on your side of a wormhole whenever possible, as it will give you first shot, allowing you to take down some of his ships without retribution. If you have the opportunity, a few starbases would also be great defenders, and would be immune to both Ionic Dispersers and Repulser Beams.

Lastly, you must put Supply Storages, or the Quantum Reactor if you have it, on all your ships. Otherwise, his Ionic Dispersers will destroy all your supplies, and none of your ships will be able to fire any sort of weaponry.

And on a much more general note: if you can, avoid fighting his fleets altogether. Instead, start raiding against his planets, and deploy some Warp Openers (Stellar Manipulation III) to outmanoeuver him. If you have not researched Stellar Manipulation, do so at once, at least up to level III/IV. If he is already "warded" from Stellar Manipulation, then you will have interesting times ahead of you.

So you should glass his planets, even if you can only destroy unbreathable worthless tiny planets. Every planet glassed will hurt his happiness, and you will eventually trigger massive riotings everywhere, destroying his build capacities and his economy. You need to glass five or six planets per turn for five turns before you reach this stade, assuming he has not filled most of his planets with troops to keep rioters under check.

In short, my advice is as follow: defend yourself as best as you can (mostly by units), and bring the fight to him by glassing his planets, and hope it will be enough to make his economy collapse.