Hit the parking lot marker to begin.
At long last, the Ark comes under attack. You don't get to fight through it, alas, but you do get to save it from the ground.
Ignore any minor LEGION ambushes along the way, and follow your objective to take out heavy offensive equipment, starting with a drop pod launcher.
Take it out as per usual. The enemies are endless, so just attempt a hack whenever you have the breathing room.
The next apparatus, three Ice Barrage Launchers, are much easier to kill, though you may want to dispose of the sniper guards first.
When you've taken out the last ice cannon, follow your marker back to the shield wall, and hack the objective'd console. There'll be enemy resistance, but unlike the drop pod launcher, this console can be bypassed.
Ride the platform up and clear the interior, being especially careful of the turrets on the upper level.
When resistance has thinned, destroy the three Wall security systems.
Destroying the last one will summon a tanktrooper and his hell-flunkies. Destroy them too, then activate the Wall console.
This saves the Ark, for the moment, and completes the mission.