Hello! My name is FlyyBug and have been a Shaco main pretty much since Mid-Season 5.
I have 160k mastery point on the champ and reached Silver 1 before the end of Season 6.
With the new changes to Shaco in the Assassin Roster Update, I found myself lost with my favorite champ. I turned to Shaclone and Pinkward for advice. I found Shaclone's experiences to be similar to mine and he raged and explained that Shaco's damage and scalings were nerfed. Pinkward was calm and analytic towards the changes but didn't seem to have a problem with it since AP Shaco wasn't really affected and was actually made better. Looking back at Shaclone, he had a problem with Shaco's burst missing the damage it use to (one of his video's explains the math in how the backstab and q lost like 200 damage.)

This guide is meant to explain how to get that burst back and how to play the new Shaco.

Hope you enjoy it!

To start this off lets talk about what they did to Shaco.

Shaco's Passive Backstab:
Shaco's basic attacks critically strike for [-80% - -55% (based on level) bonus 」critical damage when striking an enemy unit from behind. Backstab's critical strike damages as normal against monsters.
Backstab cannot happen on the same target more than once every few seconds.

This is one of the most important things for Shaco players to understand. Guaranteed critical strikes on targets amplifies your damage by 20-45% on that first initial burst auto. You can also use this to clear the jungle faster. Kite to the front and back of jungle monsters to get more backstabs off. The cooldown of your backstab passive also decreases with cooldown reduction.

The previous passive of 20% damage consistently from behind has been changed to a timed damage amplifier that scales over the course of the game, shifting his power later.
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Shaco's Q Deceive:
Shaco blinks after 0.125 seconds to the target location and becomes invisible for a few seconds. Shaco remains invisible even while casting Hallucinate. Casting Jack in the Box will cause Shaco to shimmer, but will not remove stealth.
INVISIBILITY DURATION: 1.5 / 2.25 / 3 / 3.75 / 4.5
Shaco's next basic attack within 2.5 seconds deals bonus physical damage and reduces Deceive's cooldown by 2.5 seconds. Backstab causes Deceive to critically strike.
BONUS PHYSICAL DAMAGE: 20 / 35 / 50 / 65 / 80 (+ 50% AP)

This is Shaco's bread and butter. His Q got nerfed really hard early, but is argue-ably better at max rank than his previous Q. The cooldown got hit a little but committing to using the damage refunds some of it. This is where AP Shaco got better due to the AP scaling. Also rank 4 and 5 give longer Invisibility than before.
With the removal of pink wards, this is one of his deadliest abilities and is where you will be doing most of your burst after walking right past the enemy front line straight to kill their carry, the only things that will stop you now is abilities that give the enemy true sight of you like half of Lee Sin's abilities. Red trinkets will allow the enemy to see a faint red outline of you, but you can not be targeted. AoE abilities will still damage you. You are invisible, not invincible.
Mastering the use of this ability will allow for some amazing jukes and ghost chases (making the enemy believe you went one way when you actually Q'd the other way just to sit in the bush recalling right next to their tower).
The duration of this skill lasting shorter early and longer later shifts more power later and this will have to be accounted for when invading or ganking. Some previous gank paths may not be as viable as they were before early on.
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Shaco's W Jack in the Box:
Shaco summons a box at the target location that stealths after 2 seconds, lasting up to 60 seconds.
If an enemy springs the box, it opens and causes surrounding enemies to Fear icon flee while it attacks nearby enemies for up to 5 seconds.
FLEE DURATION: 0.5 / 0.75 / 1 / 1.25 / 1.5
「 MAGIC DAMAGE PER ATTACK: 35 / 50 / 65 / 80 / 95 (+ 20% AP) 」
Jack In The Box can be damaged by champion basic attacks for a third of the box's health and are instantly killed by turret shots. Box health scales with Shaco's level. Boxes deals 50% reduced damage to turret.

Not much changed here besides box survive-ability. This allows Shaco to be able to place boxes effectively in team fights. The fear can save your life a majority of the time when trying to assassinate the enemy. Can also help lock down targets that you are ganking if timed and placed correctly. This ability also allows for Shaco to not take as much damage in the jungle due to the boxes taking a majority of the damage.
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Shaco's E Two-Shiv Poison:

PASSIVE: While Two-Shiv Poison is not on cooldown, Shaco's basic attacks slow enemy units for 2 seconds.
SLOW: 20 / 22.5 / 25 / 27.5 / 30%
ACTIVE: Shaco throws a dagger at the target enemy, dealing magic damage and slowing them for 3 seconds.
MAGIC DAMAGE: 5 / 35 / 65 / 95 / 125 (+ 90% AP) (+ 85% AD) (+ 3% - 8% (based on level) of target's missing health)

This is where Shaco got the biggest nerf and and shift in power.
Previously Shaco could use this in his initial burst, but now it must be the finisher to do the most damage. The on-hit slow this ability gives can help in keeping up with enemies that you are ganking and allow for allies to also join in on the gank. The active slow can help catch enemies that have used their escape and you need to catch up to get that last auto. Always use it in ganks whether or not you can actually catch them. This pressures them from over extending due to the fact that you are constantly reminding them of that slow and damage potential if caught.
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Shaco's Ultimate Hallucinate:

ACTIVE: Shaco vanishes for 0.5 seconds, then creates a clone of himself that lives for up to 18 seconds.
The clone explodes upon death, dealing magic damage and spawning 3 mini-boxes that trigger simultaneously, dealing damage and forcing nearby enemies to flee. These mini-boxes share the same health.

This ability has many different uses. To trick the enemy into using their ult on it *cough* Zed *cough*. To dodging almost every ability in the game (Lux laser). Split pushing like a god to surviving a team fight because of the mini box fear. Mastering the many uses can turn a Shaco player into a god and a menace. Tethers are this abilities one weakness due to that tethers do not break when ulting.

Shaco can still split-push like before, but the clone damage to towers was nerfed and his ult provides that fear utility, therefore I believe his team fight is better as he cant push down towers as fast as before.

I will now go into runes, masteries, items, and skill sequence and explain how these changes affected those.

Marks:

Attack Damage, Lethality, and Critical Damage marks:
Standard AD marks work just as well as before but now let's look at the other options.
Lethality marks: synergize will with physical damage and Duskblade. May be used if looking for a slightly bigger power spike after Duskblade, however is doesn't work with W, E, and R explosion damage. While AD adds to E damage.
Critical Damage marks: Adds bonus damage to your crits. Considering that you are criting at every point in the game these can really amplify your damage. Consider this regular 60 AD auto with 8.5 AD runes and backstab at level 1 = 68.5 * 1.2 = 82.2 damage, while a 60 AD auto with 20% crit damage runes = 60 * (1.2 +.2) = 84 damage. Small difference yes but now take this up to level 11 when shaco's backstab is maxed. Let's say 180 AD. Plus 8.5AD time backstab crit = 188.5 * 1.45 = 273 damage. 180AD plus 20% crit runes = 180 * 1.65 = 297 damage. That's a 24 damage difference and that's every backstab and thats not even considering the extra Q damage that also can backstab crit and other damage multipliers from masteries. This is all taken into account against enemy units, against monsters its an even bigger difference. This is why i decided Critical Damage marks are the best for him at the moment.

Seals:

As always Armor seals for jungling are standard. Another option that maybe viable is Scaling Health seals if their team is high in magic damage.

Glyphs:

Flat MR glyphs vs Scaling CDR + Scaling MR glyphs:

Flat MR is Standard for most but in the jungle you don't see much magic damage thrown at you. Take this if you plan to gank their AP mid or top early.

Cooldown Reduction on the other hand is essential to Shaco to survive and to dish out damage. Backstab, Q, and E all benefit greatly in order to escape and get as may crits in as you can. I take 10% scaling CDR glyphs and 3 Scaling MR glyphs to fill the rest of the glyph slots while providing a little MR for later.

Quintessences:

Attack Damage Quints add more damage to his early clear and to amplify his crits a little higher. Attack Speed Quints can also work, but you want more burst than DPS is you are an assassin.

The masteries are setup around burst with Stormraider's Surge to escape.
Notes:
Bounty Hunter vs Double-Edge Sword: Bounty Hunter takes a bit to amp up but is worth more than taking extra damage with Double-Edge Sword.
Green Father's Gift vs. Dangerous Game: Green Father's Gift adds a little damage to your clear and when ganking there are plenty of bushes to gank from if the wards are cleared for a little more damage to your burst. While Dangerous Game provides some survive-ability.
Same with Natural Talent in Ferocity versus Vampirism: more burst, less sustainability.

more on this later
Buy machete never upgrade it (pretty common with AD assassin junglers, like Rengar, but they usually went yommuus ghostblade)

can get boots of swiftness or CDR boots if you have 2 cloud drakes

more on this later:
max q for burst and utility
w second for utility
e last because is scales with the game with one point anyway
could max e 2nd for 100 extra damage but not too worth
of course R whenever available

Smite ignite for kill potential
exhaust is viable if afraid of enemy assassin or carry one-shoting you or your teammate