Here is the full list of all 12 Orbital Gear achievements.
Get the most kills in a match
Kill 10 Gears with Mines
Stop 50 projectiles with shields
Kill the same player three times in a row
Get the most assists in a match
Shoot down 10 projectiles
Destroy 100 buildings in Orbital Warfare
Kill 500 Gears
Hit two players with the same Discharger shot
Hit two players with the same Gauntlet attack
Knock an opponent out of the stage
Kill two Gears without landing