Impression from S&P was that antifighter systems only affect those on attack runs - the Portafis is a bomber.
6 dice, AP, against hull 6 - will average 1 hit. Practically, you need to throw in all 6 and hope you don't roll under 5. He then has a 1/6 chance of surviving anyway thanks to dodge.
Again, for a Raid level minbari ship, that's all of the main armament from an arc fired at 1 flight of fighters to guarantee a kill. Otherwise you risk throwing 3 AD at two flights and coming up with nothing...