Adding on a bit of information and a video guide detailing Mission 2-1 grinding.
You can do Mission 2-1: Crash Site in an average of about 1:20 (plus loading times) on Survival difficulty. Each run will net you 30,000 experience, and failed runs tend to net over 20,000 experience. This is usually enough for 2.5 levels, until you get to about level 25. It's still over a level a run at that point, though.
Take advantage of you RB roll to avoid damage. Follow the path in the video towards the objectives. When you arrive at the satellite it may take a few seconds for the objective to spawn. Interact with it and head towards the level exit. Interact with it when you reach it, even if it doesn't yet show as an objective. At this point, the stage is over, but you were so fast you have to wait for the dialogue to finish. Keep moving at this point so you don't die! I usually just ran down the left side of the map and grabbed loot.