To me MC and Garrus come from two universes of completely different narratives.
In Halo franchise it's even
possible for one soldier to beat hordes and hordes of enemies. In reality, "every swordsman's an ass, when enemies are vast". I'd call MC a mighty Mary Sue, but he's not even a human being, he's a collective emotional state of thousands xbox players worldwide beating uncountable waves of monsters every minute we speak. And I like MC that way.
But ME is about teamwork and there was no reason for introducing a superhero there. Neither Shepard nor any of her/his squad members can do alone things many FPS protagonist does instantly.
By the way, there is a thing about ME characters that was always bothering me. Don't you think that they are mostly defined by what others say about them instead of what they actually do? Many times as characters were referring to my Shepard I had the feeling they're exaggerating. Shepard this, Shepard that, like if (s)he wasn't simply doing what the Journal told

I can hardly recall one moment of independent initiative, like
"to give the Covenant back their bomb" 
IMHO even Shepard is less of a Halo badass and more of an awesome diplomat.
Funny thing, while Halo gameplay is quite unreal, the idea of
[SPOILERS] Halo array wiping out all organic life in the galaxy seems more believable (imaginable?) than a signal easily converting every organic and synthetic into a hybrid (I mean synthesis).