As far as my personal experience goes, I am not sure that you have many options.
Of course the best solution to prevent being attacked by an UFO is to counter the dark event before it takes place. It is not always possible to do so, of course.
2. Evasive maneuvers
If you fail to prevent the dark event to happen, you have a chance to be attacked from the ufo sometimes soon. When an UFO is in pursuit trajectory, you will have a warning message that offers you the possibility to implement evasive maneuvers (basically, fly off your current position and hope to avoid the fight). This attempt is not always successfull, but for me it worked 2 times out of 3 so far.
Well I understand that this looks bleak and the ground defence mission is quite difficult. I also found it quite enjoyable, but I had a fairly extensive and experienced soldiers rooster, so that might be a difference. But do not be afraid, there are no unwinnable missions in the game, no matter how difficult they look! Here are some tips that I found useful during that mission
The mission will get harder the longer you play it. So you should try to reach the first objective as soon as possible, and then try to have an all-out retreat to your base camp. The longer it takes, the more reinforcements will be dispatched
Have a lot of soldiers ready
Central will dispatch to the mission all non-gravely wounded soldiers (not sure if all, but in my playthrough I managed to deploy 9 soldiers) your reserves every other turn. Of course the more soldiers you can deploy, the easier is to defend the Avenger.
Upgrade your Avenger
In mid-game there is a room called Defence matrix that you can research. That room gives you some automated turrets to defend yourself with during the mission.