Can easily be done on the very first level and it requires a partner
When shooting an enemy it will sometimes lean and stop moving. Walk up to it and it should give you a button prompt. This is your special physical attack and holding the prompt will charge it. At which point it will attack on its own when it is fully charged. Your partner should be doing the same thing at the same time. It is much easier with a mic so you can communicate with your partner when you are beginning your charge attack.
It doesn't have to be that exact second when the both of you make contact. As long as both you hit your enemy your achievement will unlock.