If you want an experienced answer(not a fun answer):
Derelicts have top tier technology. Antimatter beams and such. You would need a ton of destroyers to defeat it at close range. However, it has no long range weaponry. Missiles from afar can slowly beat it to a pulp(and I do mean slowly), if my memory serves me right. However, if you do want to do it the fun way, you can specifically target "turrets" and slowly disable the derelict. However, you would have to leave it alone until you could afford the number of destroyers needed to do this. Like 20-30 with an armada CnC. There are no benefits to disabling it. You would blow it up anyway. But its fun. 
On the note of disabling it. I couldn't help myself so I checked the wiki. If you disable instead of destroy, it's an extra 15% bonus to research (so 25%, destroying it gives you 10%). Also, destroying it only gives you the bonus for 1 turn, disabling for 4. So, should I go lasers for accuracy? I don't want to risk missing too much and blowing it up, as that's a significant bonus, especially with the 50% research speed.