You're going to have to get very used to this game to try this.
I had a lot of very close runs leading up to the unlock which was frustrating.
Before you even attempt to be determined for this, play the game through many many times. Get to know enemy placements.
Get very familiar with the anchor system and how to use it to your advantage. Using an enemy to take out a large cluster of incoming enemies will net you big medal rewards. Continuously shoot afterwards to bump the value of the medals up, thus earning you more points.
You'll want a large medal count on every level, in order to activate the latter forms of each boss.
I personally prefer excelica, but going for high scores, I seem to do better with Crueltear.
In order to achieve 100,000,000 points, I'd definitely say you want to try to kill almost every enemy, defeat every form of each boss as quickly as possible (learn when there is an enemy or object you can take into or pick up during the battle to throw at it.), and trigger Faintear on both levels 1 and 3. (again, just medal count after the boss). Also, make sure you get to know where the actual hit box on your ship is. This will help you in a tight spot (if you anchor an enemy, there is a small triangle that appears on you, that would be the spot)
This can be achieved on easy, and it will be a close.
Bump your lives and bombs up to the max in the options, and try not to use them. After each stage you'll gain bonus points for your remaining lives and bombs (as well as medals) and you should be able to get an easy 2 million points extra (if I remember correctly).
A continue means your score is reset, so make sure you can beat the game in the set amount of lives.
Above all else, practice and don't get discouraged when you get close.
Also, going for this should easily net you the "true endings"