I just tested it in the simulator, and I'm also having the problem where the pandomini fighters are unkillable. I put 2 terran battleships and 3 zorg frigates against a single pandomini fighter. After 5 minutes, they'd failed to do any damage to the fighter. I also noticed that the fighter didn't stop shots. Projectiles and beams would pass right through it and continue on their course as if there's no ship there.
So it's not that he's immune to damage, he just can't be hit. Everything goes right through him like he isn't even there.
I glanced at the ship and race files, and found the problem. Inside pan_ftr.ini you have the following line:
MASK ships\pan_ftr.png
That line should read "MASK ships/pan_ftr.png" instead. Looks like you got your slash and backslash confused.
I changed that line of the file to what it should be, relaunched and ran the exact same battle in the simulator again. This time, it took my flotilla about 5 seconds to kill the fighter.