I've not seen this movie, so I can't say for sure, but it's worth pointing out that guard is one of the positions on a basketball team, along with center and forward.
So, one possible meaning of this dialogue would be that the coach is talking about a particular player – a guard on the opposing team.
"Keep working left" might mean that the coach knows this player doesn't guard well on the left, or perhaps that he "cheats" to the right. (At the high school level, many players will overdefend on the right side, figuring the opposing player probably isn't all that skilled with their weak hand).
So, this is one possible interpretation:
- There is a player on the opposing team, who plays guard.
- I think this player might be guarding you during the game.
- I've seen this player play defense, he likes to overdefend on the right side.
- If you can drive to the hoop dribbling the ball with your left hand, you'll "torch" him (meaning, "you'll dribble right past him, and have a chance for an easy lay-up").
That's certainly all plausible, although, without having seen the movie, it's entirely possible that the dialog means something else. But, with the context I'm provided here, that's certainly a valid interpretation, and it explains both guard and torch in this context.
"Torch" is indeed slang for "burn"; however, in the context of athletics, I think it's more likely to be used when referring to one particular play than for an entire game.
Fred was really burned on that play!
Yes, I hope he can defend better next time.