Running Man Achievement
Kill 75 enemies while in the stealth mode of Overkill.
Put your AI partner to aggressive-hold and wait till the aggro meter fills up. The aggro bar on his side will fill to the max, then another darker red bar will fill inside after that. Let him maintain the aggro for another 2 seconds, and press to enter Stealth Overkill mode. Kill 75 enemies to get this. Not firing at all while your partner draws aggro will fill the meter quicker.