This isn’t too terrible, took me about 2 or 3 games running U-Wing while actively trying to get the achievement. You need to have the repair kit and tactical shield as both of your alternate weapon kits for starters, you’ll have 6 repair kits and 10 shields to give to allies when they are low. If you find yourself out of supplies, remember that you can turn down your engines all the way underneath your teams capital ship (while your team is on offense, not available during defense phase) or you can redeploy your u-wing from your teams flagship.
My advice would be to charge up your overshields (right D pad) to 100% as soon as you deploy so you don’t get shot down while deploying and switching to max engines when you see a teammate getting low. Turn your targeting on “My squadron” (hold LT and rotate the wheel to my squad) and cycle your squadron pilots by pressing A to see how much their health they have.
I’m not sure what percentage your ally needed to be at for it to count for the achievement but it’s definitely below 40. I feel like I healed more than 10+ of my allies in the 30-40 range and didn’t seem to have any progress, but I do know the last ally I headed for the achievement was at 13% so the lower the better.
Just focus on healing your allies when they’re getting low and it should be no problem for you. Try shooting the tactical shield first then repair incase you can get progress of 2 heals for the achievement that way (I cannot confirm whether that’s how it works or not but obviously doesn’t hurt to try)