Avoid major Xenon fleets like the plague. You won't gain much from killing them because they spawn rather quickly. Focus on destroying all their undefended sectors. This is the only way to get rid of them. It's a race of who can conquer more sectors. You + the factions against the Xenon. And you have a very good chance to win that race, because Xenon sectors will go down after destroying just 1 station. Usually they don't even leave any defenders. So you can stay away from their invasion fleets and 'clean up' behind them. Eventually there are no Xenon sectors left and their fleets can be defeated for good.
If you do need to fight a Xenon fleet because they come too close to your territory try to do it when they are still in NPC controlled sectors at your border for massive rep gains and hopefully some fire support. You can seriously immobilize them for many hours by conquering a sector in their path and placing a ton of cheap protein paste factories very far away from each other. If you cannot afford that give these sectors to NPCs as they will build stations fairly quickly which slows down reconquest by the Xenon. You should have enough reputation to do that. In Mayhem 3 you don't need to reach high ranks with the factions. Reputation is mostly just a currency that allows you to expand peacefully. You should spend this currency right now to defeat the Xenon.
If you're desperate you can still unleash a deadly triple combo. Trigger an EMP to drain roughly 70% of a fleet's entire health pool, then use jump beacons to bring your own fleet in at full health, and then activate the Ring of Fire Perk. If you can't stop them with that it's hopeless.