Borderlands 2's Assassin, Zer0 offers a little different playstyle than Mordecai did in the original game. Using his Deception Action Skill, Zer0 tosses out a holgraphic decoy then turns invisible, allowing for some major sneak attacks or to get away from a bad situation. In your four-player party, Zer0 is definitely going to be your stealth and sniper character in Borderlands 2. Plus, he has a badass sword. How can you go wrong with that?
Let's look through Zer0's three Skill Trees - Sniping, Cunning and Bloodshed. Then after that, I'll offer up how I think Zer0 should be built for maximum effectiveness.
Zer0's Action Skill: Decepti0n - With Zer0's Action Skill, he tosses out a holographic decoy of himself, then turns himself invisible. While he's invisible, his next attack will do extra damage. Just be careful around explosions since he's still able to take damage while invisible. Decepti0n has a unique way of handling the damage boost, as well as the cooldown - the longer you're cloaked, the more damage you'll do; but the shorter you cloak, the shorter the cooldown. The base cooldown on Decepti0n is 15 seconds, and the initial damage boosts are +650% Melee Damage or +200% Gun Damage and +250% Gun Critical Hit Damage.
To make sure that you strike hard from the shadows, grab some skills from his three trees - Sniping, Cunning and Bloodshed.
Sniping : This tree, believe or not, is focused around Guns, especially sniper rifles.
Headsh0t - Tier 1: This is a straight +5% Critical Hit Damage boost, per level.
0ptics - Tier 1: Gives you +3% Zoom and +12% Aim Steadiness, which makes your aim move less when you get hit, per level.
Killer - Tier 2: Kill Skill, killing an enemy will give you +10% Critical Hit Damage and +15% Reload Speed per level for a short time.
Precisi0n - Tier 2: It's a straight up +5% Accuracy boost, per level.
0ne Sh0t, 0ne Kill - Tier 3: The first shot from a full magazine deals +12% Damage per level.
Vel0city - Tier 3: Grants +20% Bullet Speed, +3% Critical Hit Damage, and +2% Gun Damage per level.
B0re - Tier 3: Your shots pierce through enemies, and gain +100% Damage per enemy pierced. Also, enemies Critical Hit Locations are highlighted while you're in Decepti0n mode.
Kill C0nfirmed - Tier 4: Grants up to +8% Critical Hit Damage per level, depending on how long you aim down the sights.
At 0ne With The Gun - Tier 5: Gives you +25% Hip Fire Accuracy, +10% Reload Speed and +1 Magazine Size with Sniper Rifles, per level.
Critical Ascensi0n - Tier 6: When you get a Critical Hit with a Sniper Rifle, you get +6% Critical Hit Damage and +5% Damage with snipers. This boost can stack up to 999 times, but the stacks will decay if you haven't gotten a Critical Hit in a while.
Cunning : This tree is focused around Zer0's Decepti0n skill, and both ranged and melee boosts.
Fast Hands - Tier 1: Gives you +5% Reload Speed and +10% Weapon Swap Speed per level.
C0unter Strike - Tier 1: After you get hit, your next melee attack has a chance to do +50% Damage, per level.
Fearless - Tier 2: Gives you +5% Fire Rate and +3% Gun Damage, per level, while your shields are depleted.
Ambush - Tier 2: Gives you +4% Damage per level when you attack an enemy from behind, or an enemy that is attacking someone other than you.
Rising Sh0t - Tier 3: Each successful ranged, or melee, attack grants you +2% Gun Damage and +1.8% Melee Damage per level for a short time. The boosts can stack up to 5 times - faster weapons will stack quicker, but slower ones hold the stack longer.
Unf0rseen - Tier 3: Your Holographic Decoy will explode when you become visible again, causing damage to enemies. The Damage increases per level.
Deathmark - Tier 3: Dealing melee damage will mark an enemy for 8 seconds. Marked targets take 20% more damage from all sources.
Innervate - Tier 4: While Decepti0n is active, you get +2% Gun Damage, +7% Movement Speed and you regenerate 0.8% of your Maximum Health per second, per level.
Tw0 Fang - Tier 5: Every shot has a 6% chance to fire two shots, per level.
Death Bl0ss0m - Tier 6: An Augment for Decepti0n. You throw a handful of Kunai Knives out, with a random Elemental Effect. The knives don't take you out of Decepti0n, and can be used five times per cooldown, and also can apply the Death Mark Skill.
Bloodshed : This skill tree is all about melee attacks.
Killing Bl0w - Tier 1: Gives you +100% Melee Damage against enemies with low health, per level.
Ir0n Hand - Tier 1: Grants +3% Melee Damage and Maximum Health per level.
Grim - Tier 2: Kill Skill, killing an enemy regenerates 0.7% of your shield and grants +1.5% Decepti0n cooldown rate per level, for a short time.
Be Like Water - Tier 2: Shooting an enemy gives you +6% Damage, per level, to your next Melee attack. Melee attacks grant you +4% Damage to your next weapon attack.
F0ll0wthr0ugh - Tier 3: Kill Skill, killing an enemy grants +8% Movement Speed, +6% Gun Damage and +8% Melee Damage for a few seconds, per level.
Backstab - Tier 3: Melee attacks do +8% Damage, per level, when you hit an enemy from behind.
Execute - Tier 3: This is a Melee Override Skill. While you're in Decepti0n, and you're aiming at an enemy, you can do a melee attack to dash forward a short distance and do massive damage. The dash attack has a range of 3 meters.
Resurgence - Tier 4: When you kill an enemy with your melee attack, you restore up to 4% of your health, depending on how low you are.
Like the Wind - Tier 5: Gives you +3% Melee and Gun Damage per level while you are moving.
Many Must Fall - Tier 6: When you kill an enemy with a melee attack while in Decepti0n, you deploy an additional decoy and Decepti0n is extended instead of ending.
Building A Better Assassin
Zer0 is your Ganker - he's built around stealth attacks that do a ton of damage. Focus on skills that boost damage, like Be Like Water. While he's also your sniper character, don't neglect his melee ability - his Bloodshed tree is full of awesome skills, like Ir0n Hand, Backstab, and Many Must Fall.
From the Sniper tree, 0ptics will help steady your aim, Precisi0n will help your accuracy and 0ne Sh0t, 0ne Kill will help you get a strong first attack off. Out of the three Tier 6 Skills though, I think Death Bl0ss0m might be the most useful - elemental attacks, while in Decepti0n, that also apply Death Mark and don't kick you out of stealth mode are really enticing.
I would also grab Unf0rseen from the Cunning tree to help cause damage with your decoy. Zer0 is a character that I really can't see any bad way to build, he's a really strong damage dealer, and can help crowd control with the Decepti0n decoy a little too. And again, he has a kick ass sword! Level him up, and lay waste to your enemies.