Here is the skill build used by cloud9h1gh on the AikaNA servers.
First Tier:
Countdown - Level: 1
Leg Shot - Level: 10
Stealth - Level: 10
Second Tier:
Crimson Bullet - Level: 1
Eagle Eye - Level: 10
Sharpshooter - Level: 10
Incendiary Round - Level: 1
Ambush - Level: 5
Third Tier:
Terminate - Level: 13
Sonic Blast - Level: 1
Rifle Smash - Level: 1
Pinpoint Weakness - Level: 10
Deploy Turret - Level: 10
Fourth Tier:
Knockout Shot - Level: 8
Ultimatum - Level: 1
Predator - Level: 6
Spot Weakness - Level: 1
Roll With It - Level: 4
Go Postal - Level: 1
Fifth Tier:
Bloodlust - Level: 3
Death Knell - Level: 2
Ghost Stance - Level: 1