This is a secret achievement: "Successfully use Multiple Missile team skill in Quick Match or Co-op mode 50 times total."
Only the berserker can launch this team skill. And you have to be the one who is launching this skill. So you have to unlock the berserker first.
You need the following badges to unlock the berserker:
The Champion - The most kills in a single game.
Kill 'em all! - Kill all enemies in 1 game.
The Master Swordsman - 100 total kills as any Warrior type.
After unlocking the berserker you can add some skills to your equipment. This way you you have a higher chance to unlock the multiple missile team skill. If you have unlocked 5 skills at start you will be able to fill out all of your skill slots by reaching level 5.
Also try to add "Shorter Cooldown" that you can launch this skill multiple times in one match. Without it you will be probably only able to launch it like 1 or 2 times.
You do not need to kill anyone. It will count as long as you can launch it. Since it is a team skill you can't launch it without any help. You need at least 2 more players. Of course it is easier to play it with friends. Randoms often ignore team skills (except Sacred Song and Cheer...).