Easy "On My Mark" achievement
In Mission 2: Blue Team in co-op, as you enter the first room with enemies near the beginning, sneak to the left, assassinate the grunt, and jump up to the vent with your partner. Inside the vent, have one player take the right turn, and the other player continue straight. Each player can drop down the vents and onto an Elite. Once in position in the vents, together, countdown so you both assassinate your Elites at the same time. To assassinate an Elite, you must stealthily hit them from behind and hold [Melee]. If both players assassinate their Elites at the same time, both players will get the "On My Mark" achievement. If you fail, simply reload last checkpoint and try again.