As you play through the game fight as many enemies as you can when going to new areas for the story and do the blue portal fights to get a good amount of kills. Even with all of that you will need to grind the kills on the last boss.
Grind kills on the last boss in the game called Temperus. It has a gimmick where it will constantly summon enemies to distract you from it. Just do not attack him to much becasue when he is at half health he will summon the 1st boss in the game as an enemy and it would just take longer to kill it.