Halo Infinite

This one can be done on any difficulty and it can be done SOLO, you need to progress all the way until the mission "the Sequence" to be able to get this achievement.

Boot up mission the Sequence. Defeat the ground enemies at the start and attempt to open the door. When it does not open, it will unlock 4 Beacons chief must visit. The one we want to visit, is the one south of FOB Kilo, near Annex Ridge. Go to FOB Kilo ON FOOT or by Vehicle, and do NOT fast travel if you are doing mission replay. Fast traveling seems to mess up the mission replay. You might be able to fast travel on a fresh playthrough though. (Edit: Other users said fast traveling on fresh playthrough made it so enemies did not spawn, therefore it is recommended to NOT fast travel.)

Tip: It can be helpful to hijack the Banshees that spawn when you start crossing the bridge so you can get across the map faster. I found it was easiest to hijack them with the grapple-shot after I crossed the bridge as they tend to fly lower once you cross. In addition you may be able to Spawn a vehicle at FOB Lima, or take the banshee at Riven Gate on the North side behind a locked door.

Once you are near FOB Kilo, follow the path leading south from FOB Kilo. You do not need to clear FOB Kilo, south of FOB Kilo is a Scorpion tank that spawns.

Edit: User FQX suggested this map of the Scorpion Tanks exact location:
https://mapgenie.io/halo-infinite/maps/zeta-halo?locationIds...

Get inside this tank, and clear all of the enemies outside of the Beacon while STAYING INSIDE THE TANK. It is recommended to NOT have marines on the MG, as they may screw up the achievement. Do not kill any of the enemies while outside the tank, use the tanks cannon. After you clear all of the ghosts, wraiths, it is possible the weapon (Cortana) will say "That's all of them" or something similar, but there is probably still some grunts hiding in the general area. Drive around and keep searching and shooting all of the enemies in the area until the achievement pops. Some of them may be very well hidden and a little bit away, but they should be within the general area. Hope this was helpful. :)

Edit: Users mentioned unlocking this by using the "tank gun", so long as they made sure to start on foot from the Scorpion's spawn. I can't guarantee this will work but if you want to give it a try please let me know if it doesn't work. I see data of people saying it did work, I want to know the data of the people for whom it doesn't work, so I can gauge its consistency. If it is consistent, then we can update the solution, thank you! :D

Just a little fun fact to add to the other guide : getting kills with the Pocket Scorpion gun/EE from Outpost Tremonius actually works.

If you don't know what the Pocket Scorpion is : it's a very OP gun you can grab during the mission Outpost Tremonius (yes, the mission, you can't grab it from the open world). Now that they added the mission replay, you can just replay Outpost Tremonius, grab the Pocket Scorpion, then simply Fast Travel anywhere or Replay any Mission.

To get the achievement, you must launch THE SEQUENCE mission and eliminate all the enemies at the South Beacon with a Scorpion tank. Go to the south of the Forward Base Kilo to get a Scorpion Tank (without using Fast Travel) hidden between the trees. If the Scorpion tank is not there, get one from Forward Base Kilo. Then go to the South Beacon and defeat all enemies without getting out of the Scorpion tank.

TIP: Capture a Banshee to reach the South Beacon faster.

Here is a video to help you: