No 1: Easy. Jump. If you time it properly, the rocket will only do half damage. I'm assuming you've already been shooting at him this whole time, so you should be able to knock him down with a well-placed grenade or shot to the head.

Sniper at long range? L2 strafe erratically. Take potshots at him when you can to de-scope him while you close. Best course of action is to simply get out of sight and flank. Because the sniper in Halo 1 wasn't an instant hit to the head, you can actually do this.

All that is academic though. You are choosing to ignore the point. You can't get out of any situation in the newer Halos. You either have scissors to paper or you die.