To unlock the Munitions Maestro trophy in Anthem, you will need to complete 12 of the Colossus Gear challenges which can be found by opening the Menu > Challenges > Gear > Colossus
You will find 13 challenges available -- one for completing four matches or open world events with a specific piece of gear equipped, and a final one for completing all 12 of the Colossus challenges.
You pick up the gear as random mission rewards and from dead enemies. You can add two weapon gear and one support gear at once at the Forge. Once you have completed four missions with one set, head to the Forge and equip the next set to continue making progress towards the trophy.
Battle Cry I: Completed 4 Missions or World Events with this gear equipped
Firewall Mortar I: Completed 4 Missions or World Events with this gear equipped
Flak Cannon I: Completed 4 Missions or World Events with this gear equipped
Lightning Coil I: Completed 4 Missions or World Events with this gear equipped
Railgun I: Completed 4 Missions or World Events with this gear equipped
Siege Cannon I: Completed 4 Missions or World Events with this gear equipped
Venom Splitter I: Completed 4 Missions or World Events with this gear equipped
Burst Mortar I: Completed 4 Missions or World Events with this gear equipped
Flamethrower I: Completed 4 Missions or World Events with this gear equipped
Shock Coil I: Completed 4 Missions or World Events with this gear equipped
High-Explosive Mortar I: Completed 4 Missions or World Events with this gear equipped
Shield Pulse I: Completed 4 Missions or World Events with this gear equipped
Colossus Gear I Master: Completed all other 12 challenges