The best technique I sugguest is to be in devil trigger and have ebony and ivory equiped, and SHOOT AS FAST AS YOU CAN. This is very effective when there is a bunch of enemies bunched up together. Plus, this boosts your devil ranking from "Dull" to "Stylish!" in no time! Ifrit is mainly appropriate for this technique.
For bosses like Nero Angelo, just steer clear from him whenever you see him attack with his sword. When he tries to shoot you with those energy balls GO FOR HIM WITH YOUR SWORD. For this I sugguest either Alastor or Ifrit for this...