how can i improve this skills

[Ecstasyyyy]

Creator- How to increase acid bomb and demonstrations by 20%
 
Clown/gypsy- How to improve arrow vulcan, musical strike/throw arrow by 20%
 
Whitesmith- Increase damage cart termination by 20%
 
High priest- Holy light 20%

 

[Mystery]

By adding this to an item:

bonus2 bSkillAtk,n,x; Increase damage of skill n by x% (supports skill names)

Take a look through several bonuses: https://github.com/HerculesWS/Hercules/blob/master/doc/item_bonus.txt