Cut the door open using the Lancer's chainsaw and go around the corner to find another Declassify Mission Skull. For this section, you have to use Hammerbursts, Sawed-Off Shotguns and One-shots only.
After accepting/ ignoring the Declassify skull, smash the door open and check the alley to the right. You'll find a COG skull on the wall. Go past it and head to the dead-end corner of the alley, past the bloodstains on the ground to find this COG tag.
L. Gaffney, PFC, Halvo Bay
Go past the barricade and you'll encounter a healing Kantus. Take it out and take cover as well since more enemies will appear. Make your way to the museum and take cover as you proceed. Prioritize taking out the Kantuses as they'll revive their allies.
Once you've reached the museum entrance, you'll find the One Shot heavy weapon on the wall. Kantus and Boomers will emerge from the burrows at a distance so stand by the railing and use the weapon to one-shot kill the enemies. Lining them up together will allow you to take several targets in one shot. After killing all enemies, regroup with your team mates to complete the section.