The way i start nuking in lets say arena is:

Chain lightening button:
/assist main
/cast elemental mastery
/use 13
/use 14
/cast chain lightening

Then i press my NS (or shock if they are in range instead ofc) :
/assist main
/castsequeence NS, lightening bolt

The reason way i do it this way is that my first cast will be on 1,5 sec instead of 2,5 (without heroisem that is) and then finishing with a NS + normal bolt. Normal bolts will get more of your +spell dmg as of longer cast time and will also crit higer. So fast first nuke and higer dps on the "finish him" button