Step one is to reduce the hatred cost as much as possible. Taking Grenadier instantly takes us to 40 hatred costs, which is a good start.
Quivers can come with -5 cost, bringing you down to 35.
As it so happens, there are 3 pieces of legendary jewelry that can do the same. Those would be SOJ, Skull Grasp and Maras, each one having a chance of rolling -5 CA cost. Should you be lucky enough to find one on the AH, you're down to 20 Hatred cost.
Sounds good right? Well we're not done yet.
Step two is to regenerate your hatred as much as possible.
Cloaks and quivers can both provide an additional 1.3 per second.
Bat gives another 3/sec
Shadowpower with Nightbane adds another 3/sec
Grenades with Tinkerer adds 8 hatred per cast
Vengeance, when combined with some pickupradius, is exactly what you need to keep on going.
Arguments can be brought to forgo -5 CA cost on SOJ/Skullgrasp to use full retro nats set for 2 dicipline/sec, to skip Preparation, and use Tinkerer as a secondary generator, and having perpetual Nightbane on.
In either case, that brings you to around 15 hatred/sec, with 20-25 hatred cost CA - with Vengeance, as well as Tinkerer should you somehow get low on hatred.
That should be enough to spam Cluster Arrow to your hearts contend..