Both John and Kurt (I think) said Linda-058 was very skilled, maybe the best even, as she was able to snipe Elites out of Banshee's, one handed, hanging upside down and is one of the few able to lone wolf it. Each Spartan has his/her own game changing qualities, such as John being combat lucky, Kelly being super fast, and Kurt having an almost 6th sense about incoming danger, as well as a few others I can't remember off the top of my head.
Gamertag: DirectControlz Twinkle, Twinkle little bat, Watch me kill your favorite cat. - Joker (Arkham City)
So John walks down the battle field where tons of plasma shots from different directions clearly miss him as well as a sticky grenade whiz by him:
-Grunt A tripped as he fired causing his shot to miss.
-Grunt B didnt see John and was just firing in the direction grunt A was firing in just to look like hes contributing
-Grunt C has some sun directly in his eyes so he cant aim worth a damn and is missing John altogether.
-Elite A's Plasma rifle is acting up. Its been doing so for a while but only now does he realize how bad it is as its not hitting John despite him aiming right at John.
-Elite B is annoyed that 3 grunts and another elite are missing John so he gets pissed and chucks a plasma grenade but clearly his anger has caused him to chuck it way to hard and it squeezes out of his knuckles a second to early causing the grenade to miss John.
-Jackal A, at this point, is determined everyone is trying to shoot as close to John as possible without actually hitting him so he fires very close to john, without hitting him.
-Jackal B is confused as to whats going on so just watches with his shield up.
Look at it this way. Cortana had the choice of which Spartan she would be paired with. She picked John because he's lucky. From a computer AI standpoint, that could just mean that, statistically, this guy gets the edge in a lot of cases based on random chance.
I mean, come on, man. We're talking about a game with galactic zombies, religious alien terrorists, and super soldiers. you're worried about the meaning of "luck" in the game?
You can't have a statistical edge in cases based on random chance. In each case various factors might tip the scales in your favour but there's no quality that magically alters all those situations to work out that way.
He's just a good, versatile soldier, and the outcome of the games is based on his thought processes and actions, not some bulls*** like luck.