I have a tactic for those who aren't very good, it gets the NPCs every time and is easy to do in almost any ship, cos all you need is 2 class 2 railguns with 2* or 3* long range engineer modification to do it. Faster drives is also a nice mod to have as speed is useful (but not critical) to this tactic. The engineer mods are cheap and from the easiest guy to unlock: Todd Blaster.
How to do it...
Let's say you've been interdicted, one of two things happens, either the enemy starts firing immediately, or they hang around in front of you scanning. In either case, face in their approximate direction and boost past them until you have at least 3k distance. Now turn flight assist off, set throttle to zero, and pull up to face back toward your enemy. BEcause flight assist is off and throttle is 0, you will continue in your current direction. As soon as you almost have them in your crosshairs, turn flight assist back on and full reverse throttle (or 3/4 if you're a fast ship, you don't want to get totally out of range). You are now flying backwards at your cruising speed, facing your enemy. The 2* long range railgun mod will give you 3.5km range, so aim for that, but 3* will give you 4 to 4.5k which is very nice.
Anyway, at this point, you will notice that everytime your crosshairs go over the enemy, they lock ever so slightly, a slight effect like a gimbal. This is designed on purpose to assist with the convergence of manually aimed weapons, but the effect is that it makes railgun sniping very easy. Pop off a few shots as best you can, by then they'll be 2k away and starting to fire, use any direction of thruster that isn't forward or back (up, down, left or right) to sidestep some of the fire, boost, fire any other weapons you have as you go past, then boost again and repeat. You can stay out of their effective range for the majority of the fight, while still getting damaging hits on them. This tactic is of course immune to chaff as well. Give it a try.
The fact that they can't adapt to this tactic is indicative of their limitations. In my opinion, they could be slightly better. If I do this tactic, they should then try to stick close to me (in which case I'll give them a load of beam and cannon hell, but only if I have a tanky ship), but no, they will keep jousting until they're dead or run away (they run away a lot the cowards).