I unlocked this near the end of Kizingo Boulevard while going for the par time on Easy difficulty.
A Phantom will drop off a Wraith and you should be able to get fairly close before it sees you. Board it with X, and instead of pressing LT to plant a grenade, repeatedly press RB to melee the Wraith. On easy it only took a half-dozen bashes to destroy it.
(Like many Halo MCC achievements, this may not unlock for you on the first attempt. It didn’t work for me first try, but it’s not a matter of needing to play on a certain difficulty. Just try again if it doesn’t unlock.)