This is my technique for those assassination missions, it's very effective in a well equipped viper, I can't say for sure whether it'll work in a less equipped cobra (your shields may not be tough enough) but here it is anyway:
- Scan with KWS at the start for extra goodness (obviously!)
- Target his power plant (you can sometimes finish him by destroying that while his hull is still up near 50%)
- 4 pips weapons, 1 pip in the others
- Lasers for the shields, lasers and multis for his hull (I pack 2x Class 1 gimballed beam lasers, 2x Class 2 gimballed multi-cannons)
- Keep him between 1-1.5km by setting your thrusters to about 25% reverse
- Circle around him using thrusters spamming your chaff as often as possible and shield cells when necessary. (I pack 1 chaff and an A3 shield cell)
- Most important thing is to avoid his plasmas
- If your weapon power gets low, multis only for a while to let heat drop