How is intensity gain calculated, and how do I get a 9.9 in Classic Mode?

Specifically, how should I play in order to achieve a 9.9 intensity, while playing in Classic Mode? I’ve seen different rounds end in different increments ranging from +0.5 – +0.8, but no explanation is given as to how those point increases are calculated. Specifically:

• Does your round score correlate to the intensity increase on the next round?
• Does using a ticket at the start of Classic affect the intensity gains?
• Is there a time limit on rounds, including the bonus, in order to net the most gain?
• Do I have to hit a certain amount of damage dealt, and conversely, is there a limit on how much damage I can take?

Some things I’ve noticed include having a teammate die (in co-cop Classic) can drastically cut the intensity gain down, sometimes resulting in only a gain of +0.1 – +0.2 intensity.

At one point I started a run with Snake, starting at the highest possible difficulty, and using a ticket. I did not have to restart any rounds, and I cleared most of the bonus points in the bonus round. After all was said and done, I still ended at a 9.4, a far cry from the 9.9 needed for the achievement.