Go to a generator in any level. Stay there and keep shooting at the monsters. You will gain a lot of experience. Eventually, the generator will stop producing the monsters. Note: If you shoot the generator, it will get smaller and you will not get as much experience.

Use locations with never ending enemies to your advantage. For instance, since your character can throw their weapons, use a strategic method to reach a point in your surroundings in which the monsters cannot reach you, but you can reach them. Just keep killing them from that spot. Since the enemies are never ending use that unfair advantage to your advantage. In the long run this will allow you to prevail in gaining experience. Once you are satisfied with your skill level and experience, then you can return to completing the game.