For anyone else that needs to finish it, here's what I did based on some tips I read on Ach360pts and some other spots:
First, I logged into my brother-in-law's account on a second controller and at the beginning of every level, I would attach his ship to mine so the special weapons were bigger/did more damage. This is especially helpful with the missles as it almost doubles the volley.
Second, I didn't know it before, but the asteroids (red rocks) can be destroyed with the beam weapon for points! I would line up the shot and knock out 3-4 asteroids as well as a bunch of enemies at once.
Third, you'll want to pick up everything, including health you don't need and extra special weapons. All pickups are worth points! You may want to hang on to the beam weapon if there are asteroids on screen or you know they're coming, but other than that, grab and use everything.
Doing this, you can get 185k+ alone on Normal difficulty if you play all the way through.
You want to have 50-60k by the 'halfway point', about 100k in the second-to-last level, and you want to make sure you have at least 165k points when you fight the final form of the last boss, as it's worth 20k when you blow it up.
The first couple times I tried this, I died fighting the last boss (when you lose all 4 lives on Normal, you can continue but your score resets). If you find yourself with the same problem, then have another person standing by and when you get to the last boss, have them use the right analog stick on the second controller to help you fire into the boss' weak point, this will kill him much faster which means you don't have to dodge crap as long.