The final enemy of Mass Effect 3. Named due his enemy type (Marauder) and the fact that he has a 'Shields' bar.
During the Battle for Earth, he was Shepard's last foe in Mass Effect 3 and, concurrently, the last boss of the Mass Effect trilogy.
His origins are shrouded in mystery, it is thought he initially set out to persuade Commander Shepard to turn back in an attempt to spare players from the agony of the Star Child and from an ending many thought to be unsatisfying.
However, there was an apparent breakdown in communications and he was ultimately slain by Shepard.
Marauder Shields fights a severly wounded Shepard, who was nearly killed after a blast from Harbinger's laser wiped out all of Hammer Team.
Due to this, he can kill Shepard in a few shots and the first shot cannot be avoided.
To counter this, Shepard must perform headshots on Marauder Shields in order to stagger him long enough while preventing him from getting another attack in.
Marauder Shields' noble warning was the ending we deserved, but not the one BioWare gave us. So we killed him. Because he can take it. Because he is not our ending. He was a silent guardian, a watchful protector. He is MARAUDER SHIELDS.
Tactics: Resist the urge to shoot him at any cost, he is there as your savior not as your foe.(If you don't, you might come here and we will have to tell you "We told you so")
This tip does not apply for the Three Husketeers he throws at you before you see him, them you must shoot, else he will deem you unworthy and will not show up untill you dispatch them.
After you face your savior, you may start your next playthrough of one of the best games so far, unscarred and with knowledge that you did your best, but some fights simply can not be won.
Remember him. He is the only one who can stop the ending.