But I'm no magician, so here goes
The main problem is getting around without taking too much damage. To do that, it's best to let the Onslaught burn-drive into a nebula, while you come towards him. That takes some practice (<- this is an understatement). Once you are in his back, you have to kill him quickly, you can survive there maybe a minute thanks to heavy armor. Also he will try to burn drive away, which you should not allow (I did though). So fire the two Reapers and then three Atropos from point-blank range, which obliterates the armor and brings the bad boy down to about 1/3 hull. Then the Vulcan, which does 500DPS now, melts away his HP. The Ion Cannon keeps his engines down (more or less) and helps suppressing weapons. When he's down to a few hundred HP, you have to get out of there and only keep your Railgun in range, which will hopefully finish the beast off while you're outside his explosion radius. Killing without surviving is comparably easy, just bring more torpedoes.
During this whole process it is important to turn around a lot so the incoming damage is distributed equally over your armor and so your weapons don't get disabled.
I'd make a video, but It takes a lot of repetition and I bored of that by now. Also I'd first have to get the software.