What does the Archon Battle Frenzy buff do

The effect of Shield Wall is: you become high cover.

That's it. You don't get any sort of defense bonus, you don't get damage resistance, nothing else. On the bright side, it doesn't appear to prevent you from taking cover, either. The net effect is that you're a regular soldier in every way, except that wherever you're standing happens to count as a high cover tile.

As far as my personal experience goes, I am not sure that you have many options.

1. Prevention

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

Be FAST

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.