Chainguns suck against fighters because of their accuracy. I'm sure that 90% of the shots missed. Thats why i don't use chainguns on my BS. I prefer slow refire-high accuracy weapons, like the order turrets. They might fire slow, but most of the shots hit the target.
I don't remember the stats of the wraith, but I've fought it a few times with nb/sb allowed, and it didn't take me more than 10 mins to kill it (depending on player skill). i fought it once with my rh bomber (i killed it in less than a minute thanks to dual novas), and a few more times in raven / sabre.
(If you find any mistake in my English, please let me know via a PM)
(Really, I speak terrible English, so please, tell me if I make mistakes. I'd like to improve it a bit )