Loadout:
Sniper Rifle / Vektor Rifle
Sword

Squaddie:
Snapshot (take a sniper shot after moving but has an aim and crit penalty)
Stillness (has a very low detection range and doesn't reveal with squad, can re-enter stillness twice per mission)
Slash
Slice (attack an adjacent enemy with your sword for 1 action)

Corporal:
Death Mark (Mark a target granting allies +25 crit chance against them)
Blood Trail (Deal +1 dmg agianst targets that have been woudned this turn)
Implacable

Sergeant:
Shadowstrike
Target Definition (Targets spotted by the assassin remain revealed even through fog)
Shadowstep

Lieutenant:
Deadeye
Silence (Take a shot that will not break concealment, has a 4 turn cooldown, needs 2 actions)
Blademaster

Captain:
Soul Harvest (kill shots increase crit chance by +5 up to a max of +20 per mission)
Infiltration (+25 hack & not revealed by advent towers)
Slice & Move (Can move and slash an enemy for 1 action, replaces slice)

Major:
Banish (empty your weapon clip into an enemy)
Mute (Passive, Gain +2 charges of Stillness per mission)
Merciless (+1 dmg and +15 crit chance with swords)

Colonel:
Annihilate (upgarde for banish, when a target is killed move onto a new target until ammo runs out)
Vanish (Move to any tile within squadisght, ends turn and uses charges of stillness up)
Lacerate (A sword attack that has 100% crit chance)

GTS:
Shadowfire (replaces standard shot, kills made with this perk do not break concealment)