GUNDAM OPERATION TROY

I can confirm that this does not have to be on one sitting. I loaded up mission 3 and just melee killed 10 MS.

To melee press the B button to bring out an axe looking weapon the his keep pressing B to use it.

Happy hunting and I hope this helps.

I managed to get this achievement on the E.F.F. campaign on MISSION 05. You will start the mission already in an MS.

Play on EASY difficulty and simply run (hold RB) towards the enemies and press B to melee them. The enemies will be defeated in just one or two hits.

Don't worry if your MS is destroyed. Simply reload the checkpoint and carry on meleeing the enemies.

Please comment if you vote negatively. I will edit my solution to make it better.