This is my first build post.
At first my english is not very good, i'm sorry.
And i don't know yet how to do a nice post.

I think there is 2 way to play with Caitly:
- first one: Attack speed to do more headshot.
- 2nd: Attack to have a heavy damage with your ultimate.

This 2 way to play Caitly must eable caitly to be a "tank killer", i mean be eable to kill rammus in team fight (high armor penetratio).

And my build is the 2nd.

Exhaust/ghost:
I think this is 2 best summoner spell to be eable to reach a good masteries tree.

She is pertilary strong agaist: (1V1)
Mele carry
Tank

Team fight: She is like ashe or Miss Fortune, hit hit, pillover peacemaker, hit... ultimate
And caliber net to run away.

Trap is hard to used, you can use it as ward to scoot a brush. Or put it close to a turret, if enemie try to diving under your turret this is a good trick to have a easy kill.