Explodemon

You will get this while going through the first level. Anytime you kill an enemy, a red explosion cloud will appear from it and you will collect it. If you need to, you can even camp by a spawn machine and continue to kill them and fill up your bar on the left. As soon as it is filled for the first time the trophy will pop. You have to have this full to get an S ranking so you should be going for this in every level.