This one isn't as hard it might look initially. First complete the DLC itself once on any difficulty of your choosing. Then finish the Son of Sparda mode to get Heaven or Hell difficulty unlocked. By now, you should have a lot of kills for the trophy but the way to complete this easily is to simply play Heaven or Hell mode until the trophy pops. You can either farm Mission 1 on repeat or just play through the full DLC on the difficulty setting if you want variety or find it to be tedious. On this difficulty setting, everyone and yourself will die in one hit so the best tactic is to just run through everything spamming Summoning swords (Vergil's projectile attack)