Something i found is that this trophy/achievment is easiest done with the second megaton. The reason being is that you get the keycard for the D.I.E. Shockwave remote which lets you get the D.I.E. Shockwave wonder weapon. The way the weapon works is holsing the ADS button uses a suction/vacuum attack that charges the weapon and can damage multiple enemies (see where this is going?) And the fire button makes the gun shoot a ball of energy that can easily wipe hordes but needs to be charged.
Using the weapons main fire split the megaton and only use the weapons charge attack to damage them both, that should kill them both at the same time and easily unlock the trophy.