I’m currently on Level 51, beaten the final boss and still only on Laser Rank 5, what did I do wrong?
You’ll need to collect all 12 D-Units that are scattered throughout the world to attain laser rank #6 
Part of the fun is exploring and finding them yourself… it is a beautiful game. However, there are many online %between%guides available that’ll tell you where to find them…
Happy hunting!
Lance’s Panzer Dragoon Saga guides explain where the D-Units are located if you get stuck:
thewilloftheancients.com/categories/secrets
Now the secret link in my post will never be found!
Haha, stealthy.