Patched. See comments for new solution by
Oi RoYaLGame Mode : Breakthrough
Specialist : Boris
Gadget : Repair Tool
You're playing this on Breakthrough so you can hang back to work on this. Very simple:
Step 1 - Using Boris, deploy Sentry Turret
Step 2 - Melee your turret once
Step 3 - Repair turret
Step 4 - Repeat
I think it took around 10 minutes for this, but could be wrong.