This achievement can be unlocked in both the campaign and free roam. I unlocked it during Sequence 2, Memory 1.
To trigger a reverse boarding, you need to be in open conflict with an enemy ship (I achieved it with a brig - a mid-sized ship, similar to the Morrigan). Position yourself so that the enemy ship can ram into your side, and if done correctly, the Morrigan will shake and Shay will stop manning the ship.
Once Shay recovers, head immediately to the point at which the enemies are boarding from. There will be ten enemies to kill if done with a brig, and your crew will help you defeat them. That said, your crew can be killed, so ensure that you deal with the Redcoats as swiftly as possible to avoid losing too many crewmates.
Once you have killed all the enemies, you will have to choice to repair the Morrigan or to salvage to ship. Make your choice, and when you return to the open seas, you will unlock the achievement!
I have included a video guide too, should you prefer. It shows the type of ship you can achieve this with, as well the rough angle you should be at to trigger the boarding.