these xenon probably had the point singularity projector
you basically have to micro your positioning to try and pick these Z or I off one by one with your entire force at once. They are super strong and there is little room for error. You just can't send a fleet to attack them and must always carfully manage these battles by splitting your fleets to trick collision detection, wait and gather them first, make sure to attack just one side of the xenon turrets and such.
All this only makes it a slog because Mayhem 3 ramps up xenon strength over time. And it goes completely overboard in the final OCV waves You basically need to grind for days before you can stand any chance. The NPCs can't help at all. It's only you who is slowly constructing standart ships one by one with resource investment vs. the Xenon who are quickly spawning waves of powerful ships for free. If you don't construct massive fleets ahead they simply overrun the galaxy.
The truth is that most playthroughs never reach the OCV. I mean who really terraforms 10 planets? You get maybe 4 planet boni which are really good, so you get these. The rest is useless and you got no reason to advance further because you know that the OCV taking over all these NPC sectors would make it a huge grind. You need to conquer the entire galaxy first so that you can extinguish any OCV wave in its infancy. So you want to do that first, but it gets boring and then you start a new run.
For slightly more reasonable Xenon you can try M3R.
And soon there should also be a Star Wars M3 where i tried to fix all these pacing issues. It has been made especially for Mayhem newcomers. The Xenon there are MUCH weaker and don't scale up over time. The OCV stage gets triggered earlier, but is also weaker and the factions should survive.