It really depends on the rating and loadout of the ship.
I've killed some extremely easy, I remember one didn't even have shields at all.
Others can be really tough, like one that was pounding me so hard my shields had dropped from full before my shield cell was able to activate again.
It can be tough to stay behind an Elite NPC, so if you struggle, go to at least 2.2km away and pound from there flying backwards.
I suggest getting a plasma accelerator, once the shields are down it really makes short work of any sub-system.
You can even shoot from 2km+ if you fly backwards in a dead straight line - they often follow you in a dead straight line too meaning the projectiles should still hit.
Direct hits on the power plant will destroy it in about 5 hits, that's 15 secs at best - no other gun will match that.