This sounds difficult but is fortunately basically free after you've finished the campaign.
During the Shield Protocols mission chain there is a mission called Secrets Within. One of the objectives during that mission is "Reach Shield Bunker." No enemies spawned for me during this objective.
All I did is follow the radar as fast as possible to the location and opened the door, at which point this popped.
The only possible consideration is I ran around the whole map at the start of the mission getting the optional chests and objectives, so it's possible I'd already cleared this area prior to the objective. Would recommend doing the same just to be safe.