You will need a game with at least 1 other player to earn this achievement!Apparently, this also works if you revive the same person twice in one match, so this should then also be possible with 2 persons.
First, you will need to get the Ragnarok DG-4. To do so, you need 3 parts. These parts can be obtained in any order.
Part 1: Dropped by a Panzersoldat, after you kill him. (he usually shows up in round 12-13) He also shows up if you are doing the easter egg, after you've done the whisps for the first time and you went back in time.
Part 2: Put on the Death Ray (electrical floor trap). When the trap stops working, look into the air to see where a blue part has spawned. Then use one of the 4 wunderspheres to pick the part up. Depending on where the part spawns, you will have to decide which wundersphere you must take.
Part 3: Go to the Rocket Pad (by using the teleporter, which is located in the same area as the pyramid and the BRM). Wait until the rocket starts launching, then activate a switch located underneath the rocket pad (located on ground level, next to some stairs leading to the platform with the rocket and the location for the Pack a Punch machine). Now quickly head back to the Teleporter. You will see 3 lights on the left side of the teleporter. Wait until all of them are green and then hold
to activate the console. You will see the last part laying in the teleporter.
Now go to one of the workbenches located across the map and craft the Ragnarok DG-4 (and pick it up). I usually craft the Ragnarok in the church/console room, you could also craft it next to the pyramid.
Have 2 of the other players down themselves, next to each other. Press
+
at the same time to hold the Ragnarok DG-4 in your hands. Then you can press
to put the anti-gravity trap down, somewhere near or on top of the 2 players. Then revive both of the players, and the achievement should unlock. If you are doing this with 1 other player, have them down themselves, put the Ragnarok down and then revive them. Then keep killing zombies until your Ragnarok is fully charged again, and revive them again with your Ragnarok down.