Follow the path and make your way to the stairs leading to the bridge. You'll find the Declassified Mission Skull before reaching the bridge. This Declassify mission will make your squad take Enfield Bridge with reduced ammo, no longshots and more Reavers to fight.
You have to use the covers to flank the machine gun emplacements and take out the grubs. After clearing the emplacement, more reavers will appear. Take cover and recover the ammo the enemies will drop. Try using the machine gun as well when it's not dangerous. Taking out the Reavers shouldn't be that hard; either concentrate your fire on the riders or its glowing underbelly. Another wave of enemy reinforcements will arrive from the other side of the river so be careful as well.
Once you've reached the center of the bridge (marked by a gold, circular railing) continue to the broken part in the middle of the bridge and turn around to find a COG tag.
W. Fellhofer, PFC, Hanover
After collecting the COG tag, take the path to the right to engage more Reavers and enemies. Remember that you can fall back to the gazebo if ever a Reaver lands in front of you. Clear the bridge of enemies then continue inside the academy to complete this section.