great khan, any tips?

[Zixky]

fighting them at the moment it's going... well I suppose they started at teh other side of the system so they are rather large every time i push them in they start to push the other way, it's a game of cat and mouse and can't seem to catch them.

[Xobra]

Git Gud :)
As a more serious tip: If you can't win at all, surrender. You'll become a Satrapy, get "protection" in form of a Void Dwelling, you gotta pay 10% Energy, 20% Minerals, and 30% Naval Cap, but he won't attack you. If you betray him, you won't get another surrender chance. The Khan destroy outpost, not just conquers them.
Sit back, watch him burn and conquer the galaxy, and if you are fast enough, build outpost in system he targeted near you, as they are free to take after he destroyed the outpost there.

If you don't surrender, fortify borders, and let your Fleets sit there. Don't go offensive, unless you got bigger, better guns. Retrofit ships to counter him. The Khan dies through an event, if nobody can destroy his fleet. (10-20 Years, assassination, flu etc).
Once the Khan dies, the Horde will be unavailable for 10 Years. After that, the Horde splinters. If there were Satrapies, there is a chance, that a new Federation is formed, and all Satrapies can join. The Splinter Empires are mostly hegemonist, if the Federation isn't formed, and are like normal Empires in terms of strength. Although some can have more Planets and Void Dwellings as other (I had a splinter Empire with no Planets, and just one Dwelling once).

If you want to kill the Khan, you need to defeat him twice. The 1st time, he escapes and come back around 10 Years later with a better fleet. If you beat him the 2nd time, he'll die, and you get the achievement.