AP shaco is so underplayed but one of the funnest characters to build. there are alot of hard hitting combos that can come from going AP that crit/AD shaco can't always compete with like the end game damage of Two poison Shiv, and the trap burst you can get from you jack in the box's

Shaco can take top in 3v3, but otherwise you're going to wind up with a lane partner most likely. farm minions. and try to Zone with jack in the box and deceive to scare the opponents back to turret. once you hit level 3 you should be able to inflict a great deal of damage to scare/kill early game. once you build up enough gold for your boots and mejai's you should definately start moving in for the kills. you want to use jack in the boxs ALOT
Once an enemy champion is under the fear effect. you should deceive in and hit him from behind for the passive damage bonus. Then continue to attack for as long as they're in you're Shiv range. once they get too far Ghost is an option, but if you keep Shiving them they shouldn't be able to outrun you without ghost/flash.

The reason the Mejai's is stuck in there so early is because shaco is able to pick up a large amount of kills, and seriously Weather it's due to stealing kills or not. the Mejai's will benefit you late game if you survive. Which ghost and Decieve will help you do. The double Deathcap at late game is almost unstoppable with Rylai's hp boost, and lifesteal. This can also keep Survivability up.

Please note that Mejai's is not always the most reliable. BUT at it's full stack (20) it grants 160 Ability power and 10% CDR which packs you with alot more damage.

Your Shiv attack has lots of damage when building AP so because of the small cooldown, it can be spammed for last hits. Jack in the box can be great for both Zoning enemy champions, and picking up minion kills. Using his Passive on minions also brings up the damage for faster minion kills.

I'm going to say right now that AP shaco is not the best for ranked games UNLESS they are jungling. in which case makes you less vulnerable and gives you a quicker way to build up so you are as powerful as you need to be against Ranked summoners. Jungling builds would be go pretty much the same except you would want to exchange a summoner ability for Smite and Mejai's can be bought more mid-late game letting you increase you're power early.

So basically Shaco just grabs lots and lots of kills, and lots and lots of Damage throughout a match. Keep in mind AD shaco is a bit more reliable, because crits and high ad+as is a great way for him to go. But Ap Shaco is so much fun, and is definitely worth your time to try out. Enjoy <3