As of Beyond Light, the Leviathan raid is no longer in the game, so Bungie changed the requirements of this achievement to: Complete a Nightfall strike on Grandmaster difficulty.
If you didn't get to complete the Leviathan raid on Prestige (an amazing experience, I might say), getting this might be less or more complicated depending on how much you enjoy harder content. Nightfall rotations change every season, but usually there are 6. In the current Season (Season of the Chosen) the strikes are:
-Warden of Nothing
-Devil's Lair
-Arm's Dealer
-Proving Grounds
-Fallen SABER
-The Insight Terminus
Grandmaster Nightfalls are by far the hardest PVE experience currently in the game, but with enough practice and a coordinated team, you can easily complete even the harder ones. You need to be at least 25 power levels less than the difficulty cap of the Nightfall (in Season of the Chosen, at least 1325, and Grandmasters are 1350).
However, considering the rotation of different strikes, some will obviously be easier than others. This season, the Arms Dealer is much easier than all others, and should be completed even without engaging much with the final boss if your team use the right tactics and loadouts. Other easy strikes are Lake of Shadows and The Insight Terminus. Some not so hard ones are Broodhold and The Devil's Lair.