This is done on "Long Night of Solace". At the start, you will be protecting the ship from the enemies that are attacking it. After this, you will see the long purple Covenant Ship that you have to attack. Immediately go behind it and start shooting rockets at it's engines. Slow down by holding down on the Left Analog Stick to not approach the ship so quickly. After this is done, start taking out the enemies that they send out to attack you. Banshees can easily be taken out with the cannon, but the Seraphs are a different story. To easily take out Seraphs, use the cannon first to take out their shields, then hit them with one attack of the missiles. This will make them much easier and faster to deal with. Once there are only 5 enemy ships left, they will show up on your screen with red arrows pointing where they are. Take out the remaining ships to complete this.
Thanks to Sidedish120 over at x360a for the description.