From Halopedia, the Halo wiki
The Brick Wall medal is awarded in the Halo: Spartan Assault Flood missions for blocking ten Flood explosion or infection forms with the Z-90 hardlight shield before it depletes. It is represented by a blue shield with the hardlight shield symbol on a brick wall.
This medal is one of the most difficult Flood Mission medals to earn. The most effective tactic is to play Mission 5: Power Net, retrieve the armor ability from near the lower right objective, then stand directly in front of one of the entrances from which infection forms spawn.