On that system:

What happens when your special technique is to boost your speed (like Ni-Mu) or strength (like Artoria)? How do you balance around that?

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I feel like a level system like you have would work really really poorly in WotT with just how bizarrely people's power level fluctuates depending on the needs of the story.

For example. Ira vs Alexi: We've established Ira's about 800 to 900 times stronger and faster than Alexi, for her to slightly lose out to Alexi in power and speed when Alexi is multiplying her power and speed by 1000. Then, turn around, and Ira only slightly outspeeds Syphax. Is Syphax 600-700 times faster than Alexi? Yet, earlier, Ira was able to move faster than Henry's eye could follow, and previously Henry was the fastest demonic character. Is Syphax also able to outspeed the entire early Demon crew to the point they can't even see him?

Obviously no.

Whenever we have an actual fight show up, the character who is faster is only slightly faster. The character with a tougher defense is only slightly tougher (Hi Zerry). So on, so forth. Everyone always gets blurred towards the same point so the fight is more fun to watch instead of. "I have more defense than you can possibly ever break. I will stand here and shoot laser beams out of my eyes, but you have more agility than I could ever match and will dodge forever."