Find a location where enemies spawn in groups of two or three. Put a Legion on active attack mode and let them kill those same enemies for hours. This works if your Legion is strong enough, but check up every ten to fifteen minutes to make sure your health is still the same. If it has changed, check more frequently to make sure you do not die.

Let your Legions take the kills and do most of your dirty work. You will get double experience. Also, press Force Attack if you are in a tight spot.

For more challenging play, but better experience, when a generator keeps making monsters reappear, just stay there and kill those monsters over and over again, but don't attack the generator.

Go to level 6 and fight all the way to the second stage. Use the Malice and keep using the Force Attack (Triangle). Press it repeatedly after there are no threats around, except for the heads with the spikes that cannot die with conventional attacks. Do this for a while to gain experience.