From Halopedia, the Halo wiki
My Clothes! is an Achievement in Halo 3: ODST. It is awarded for killing 10 Brutes, Jackals or Engineers within seconds of disabling their shields with a charged Plasma Pistol shot. It is worth 5 Gamerscore. Despite the achievement description stating that the achievement must be done with Brutes, it can also be done with Jackals and Engineers. This is likely because the game requires ten EMP Kill Medals to unlock the achievement, and they can be obtained from Jackals and Engineers. It also counts if the enemy's shields were stripped away by a Power Drain.
It is represented by a red and silver signpost with a Plasma Pistol in front of it.
It is easiest to do this on the first flashback level, Tayari Plaza on Easy difficulty, as there are plenty of Grunts with Plasma Pistols and they are often accompanied by Brutes. The achievement can also be easily attained in Firefight mode.
Using plasma pistols from Grunts or Jackals, fire a charged shot to remove the Brutes's (or jackal's) shield, and kill them with a melee or a headshot from your M6C/SOCOM. Repeat until the achievement is unlocked.