Once you are done with everything else in the game, you probably would be level 20 or so only and need to grind this. Here's a fast way to grind. The idea for this method came from a forum post by K4rn4ge, but it has been modified by yours truly for better efficiency.
The original forum post by K4rn4ge, I will quote here:
There is also a quick way to level up to 50 to make the grind a lot less painful. Simply put like 390 enemy ships in one pile on top of each other in the level editor and add a few powerups nearby. Load the level and play it on legendary difficulty 10 for a bunch of XP.
I noticed that killing enemies didn't actually increase my experience at all, and the amount I gained differed every time I replayed this map. So, I looked at how I get experience, and it turns out it's from random item drops. Naturally, my next line of thought was, would I get experience if I place bunch of power-ups on the map? The answer was yes! So, I modified the method by placing 1 enemy, and 399 power-ups. With this you get around 10-20 experience per play, but since you are not fighting at all and you can hit restart right away, it only takes 10 seconds per try. I noticed I could level up once every 5 minutes, so you can go from level 20 to 50 in about 2.5 hours.
Here's a video to better show how it's done:
Please note that this method will also give you a lot of money. You will see I have 30m when I get my achievement for level 50, so you can get Investor. Also, finishing the level is guaranteed S rank, so you can get the achievement for S rank.
However, as the method does not kill any enemies, you may miss out on 10,000 kills if you rely solely on this. So, I advise using K4rn4ge's method for the kills on easy 0 difficulty, and then leveling using this method to finish the game's end game grind quickly.