General

To expand on what above said, Arrow Rain's scan range to spawn on monsters is dependent on the direction your character is facing from the monster's center.

Gollux is one of those weird monsters that has a fixed center, so unfortunately Arrow Rain will only spawn at that center, shown in the below picture.

So at the gem phase you would want to occasionally swap sides from where you're attacking the gem to take care of the monsters at the bottom.

Here's some quick graphics I made, click between the two and you'll see what I mean:

http://i.imgur.com/vDQ5HOk.png
http://i.imgur.com/1e0iJyq.png

Any monster within Arrow Rain's scan range will spawn an Arrow Rain on them.

And yeah, doing around 18m on Hurricane sounds good to me. Might be less. The best way to find out is to simply try!

probably around 18m lines. use arrow rain on gem so it mobs for you, you can then focus arrow blaster on gem as well.