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Um hum so the walkers are sectopods?
I killed 4-5 of them and i never got an autopsy option in my reasearch tab.

There are no easy way to take down an etheral base.
Try to take some tanks to absorb fire.
Or hide around the corner walk out take your shoot and walk back in.

Try to take the best guns in the game (blaster,Hevy plasma,hevy laser).
If you still have problems use explosive ammo but remember that this can destroy most of the merchendise.
always take HE packs and alien nades.

If you use the skyranger pack it up with troopers, the same with the lightning.
Or you can use Avenger with one or two plasma tanks.