Word of advice: don't play on hard or easy, the game is balanced for normal difficulty and the two other levels are a bit buggy at times.
While I played through TF a lot of times I had played thorugh XWA only once when I was in high school before attempting it last year and remeber it being stupidly hard on easy. Last year instead I started on hard but dropped to normal and for some reason it got much easier than I remembered and even got all the Kalidor Crescent upgrades thanks mostly to the ability on normal to disable ships engines with the shields still up so they can't flee when you drop their shields, believe me it's really easy to rack up bonus points by downing capital ships this way.
When I first played the game (many years ago) I ended up skipping that mission to get to the second training mission where you dogfight against growing numbers of TIE fighters, and that was so much easier. However, having recently tried playing it again I finally beat the Y-Wing hit-and-run training mission having followed some advice. Drop all energy so that it goes to engines, and race towards to the convoy craft. Dumb fire missiles when relatively close to the convoy craft so that you cause the pulling (or pushing on some) craft to become disconnected from their cargo. Then flee the area and hyperspace back to the fleet to re-arm on missiles. Return to the mission area and destroy any remaining dropped cargo at leisure. Still took me a few attempts but I was so happy when I beat it. :)
It's 2019 and still the same problem... That escort trash me up in seconds... I'll try your metod but it's sounds more like a solution to a adventure game than a filght sim. Only one way to do it :) Ohhh and yes, I play on easy :) I watch some yt videos and not everyone have this problem, the wingman take care of escorts. I hope there isn't mamy missions like this. Tie Fighter wasn't that hard (original campain on easy :P)
I never really had the need to dumbfire, you can do it from a distance if you choose the tank containers since they are the most fragile and lock doesn't take that much time.
Also, play on normal like I said above, hard is not worth it.