Splitgate

You must earn a kill using portal placement. To do this, an enemy with full health must go through your portal, and then fall to their death as a result.

The best maps for this are Olympus, Highwind, Crag, and Abyss, as they have out of bounds portal pads on the outside of the map.

Place one portal on these pads, and another on the map and if an enemy passes through it, and falls, you will earn this.

This type of kill has a special icon in the kill feed, and the achievement does not unlock until the end of the game.

first, get on the map 'Highwinds', its by far the easiest. Next, put a portal on the wall in the sky in the back corner, and then place your other portal in convenient spots and hope someone goes through it, this is a very situational achievement, which can be annoying. Refer to the video below for better clarification of where to put the portal. Again, this is entirely luck based, but this is the easiest/quickest method

How I got it

This achievement requires you to make an enemy die as a result of falling to their death with your portal.
The easiest map I recommend for you to do this is Olympus. Just make your way into any of the side bases and go to the upper part, there will be a zone far away at the side where you can place a portal leading directly to the bottomless pit, place a portal there and place the other one by your side.
Now camp for a little bit until an enemy comes and attacks you, when they do take the portal, if you are lucky like I was, the enemy will follow you, you most likely will kill yourself but you'll take the enemy to the grave with you and as it is usual here in Splitgate, the achievement will pop at the end of the match.

Here is the video of how I did it, but keep in mind that since it is a replay some things are not how they should be, enemies are showed blue on the crosshair as well as their portals, and the game doesn't you show the medals on replays, but I can assure you it was an enemy the one I killed and the game did give me the "Portal Kill" medal.
Also, as you can see I shoot at the enemy before the drop kills it, this is not necessary, but the achievement will be unaffected if you do it.
Good luck :)

I made a video on this as well.
https://m.youtube.com/watch?v=N4wW3QY94t8
Just another way to get this achievement. If you are having trouble with this achievement, use this video as an example! You can use any of the portal panels high above on the map Highwind. Game mode, king of the hill. Then use your other portal on the location, in this video. Most players gather around on the hill point. Chances are, one will go through the floor portal and end up coming out on the higher portal.
You can be sneaky and use the sniper tower as well to your advantage. Place a portal the panels above, opposite side of the hill, and one on the floor of the actual hill point. With a bit of luck, the player will fall off the map, giving you the achievement. Good luck. Hope this helps to give you an idea.
Note: you must get a kill by making them go through the portal and fall off the map. You wont get the achievement if you kill the player after they go through the portal by shooting at them.