After some short testing for the purpose of New Game optimizations.

Each level of mastery seems to give an, on average, 30% damage increase from the base damage at level 1.

It takes 3 skills at 100% to raise to level 2. I think 5 to level 3.This means Level 2 Mastery might be attainable and useful for the run.

A notable exception is Whirlwind, which seems to not have it's base damage increased.

Useful combat skills and sequences for Shadow Whip seem to be Somersault Attack for quicklyu avoiding blocks, DA launch into Guillotine loop for evasiveness and damage, and DA counter strike whip for damage option. Sustained Area Attack seems less useful until you get Infinite Blizzard going to control the enemies movements due to lack of stun.

Of the options to build Mastery, DA Launch into DA aerial combo are quite quick to level in comparison to many other options. AA on the ground can be fast with large numbers of surrounding enemies. Counter Attack and Somersault Attack tend to build quickly as well.

Stolas Clock accelerates EXP gain based on an unknown factor atm, but you gain EXP every hit. This is possibly based on damage. This might be useful for building EXP while controlling enemies.