This game mode is only available for a limited time. At the time I am writing this, this is only a day away from ending until it comes back again (date uncertain).
Find an available match of Ascent under "Play" at the top of the main menu. If you are going for the achievement, spawn as the defending Austro-Hungarian team. The person being killed by the rock needs to be on the attacking Italian team.
Pick a spawn point, preferably one that is closer to the attackers' two starting points. Once spawned in, you will see markers on the map of these little circles that look like ammo cans with rocks falling out of them. These are the locations of rock piles. The person going for the achievement should find the closest one, press whatever key or game pad button you have to pick things up and look over the ledge of the mountain face.
Italian player: climb the mountain towards your position and stop about 50 to 100 feet right below the player with the rock.
Player with the rock: press your fire key above the Italian below. This may take several throws or tries. The achievement should immediately pop once the rock connects and kills the other player.
Here is information about the game mode from the developer:
What is the Ascent event?
Ascent is a limited-time special game mode in Isonzo, where players either scale the Marmolada mountain's cliffs as Italian attackers or defend against them as Austro-Hungarian defenders. With unique mechanics diverging from the main Offensive mode, it offers an immersive experience of alpine warfare, challenging 32 players to conquer or hold the mountain, and for everyone to earn a special event medal upon completion.
How long will the event last?
The event is available for a limited period of time only. (November 14 - November 26)
Why is the Ascent event not available permanently?
It's a limited-time mode likely to repeat once or twice a year, similar to previous special modes like the Christmas Truce and Wolf Truce. We do this partially to stay accurate to historic events and to have people come together annually to fight for the same goal.
How do I take part in the event?
Everyone that owns a copy of Isonzo is able to take part in the Ascent event, players can join the separate game mode available on PC and now for the first time in WW1 Game Series history also on console.