Code:
#showtooltip
/cast Rapid Fire
/cast Kill Shot
/cast Chimera Shot
/cast Arcane Shot
/cast Aimed Shot
/cast Steady Shot
As long as both Kill Shot and Chimera Shot is on cooldown and therefore fails, the GCD is not triggered. This enables Arcane Shot to be used.
And ofc when Aimed Shot, Arcane Shot, Chimera Shot and Kill Shot is on cooldown, Steady Shot will be cast.
I recommend that you bind this to your mousewheel and just roll it.