/castsequence reset=15 Serpent Sting,Arcane Shot,Steady Shot

add the cast times of Arcane Shot,Steady Shot then add more till yeh get as long as yeh Serpent Sting

it gonna look something like this
/castsequence reset=15 Serpent Sting,Arcane Shot,Steady Shot ,Steady Shot ,Steady Shot ,Steady Shot,Arcane Shot,Steady Shot ,Steady Shot ,Steady Shot ,Steady Shot

Arcane Shot has a 6 second cooldown