There many ways...
Easiest way would be per function what checks the Class id , and if it's the class id of archer and he use a skill, the function create a 0,6-1sec CD on all Skills the Archer own.
Easyiest way with a IF and a LOOP after it.
But dont forget to refer the new function in the other skill functions, you have to connect them all together.
A harder but better way is per Sourcecode of Client.