Multiple Missile requires 3 players (including the Berserker player) to activate, and you'll need to do it 50 times.
Team Skill executions towards this quest can be gained from either Special events, Quick Match, or Co-op.
If you have these buffs, equip them on your Berserker: Cheer+, Rush+, Charged Attack+ (or Smash+), and any of Rocketman+, Stone Throw+, or Super Guard+. You'll also want Team Skill Exp Up for your helmet and an accessory with Conserve Team Skill AP and Shorten Cooldown Lv2 or better. You should be able to get these buffs as rewards by playing the various modes for a bit.
By having 4 skill+ buffs on your gear, you decrease the number of levels required to max out your skills, and by doing a nearly-full Cheer at the start of the game, you should get Multiple Missile just about every single time right at the start of the match, and if not you'll get it at level 4. The Conserve/Cooldown accessory will allow you to use the skill as often as possible. If you play in Quick Match, and are playing a particularly long match, you can spawn stronger for 3000 Happy Stars and choose 'Shorten Cooldown Lv4' to drastically decrease the amount of time between uses and make some rapid progress on this achievement.
As a bonus, this gear combo will help you advance on the 'They Had It Coming...' achievement. You may choose to not have a hat skill+ buff to increase the chances of getting the Berserk skill, but this will delay your progress on this achievement, which is more of a grind.
For quickest and best results play with friends. If you're playing by yourself, be sure to command a few friendly bots to 'Follow' before attempting your team skill.