This guide is for building an evolutive jayce.
Jayce as you know is quite squishy in early game and has not brutal damage.
However, this guide is for looking your opponents face when you kill him with only 3 auto-attacks.
This Build give you the chance to be a god in lol, and if someone don't trust me is cause of didn't have played versus me ^^
The main damage you will do is with your W (in cannon form ), that spell at level 5 increase your damage 130% and your attack speed to the limit with your 3 next attacks.
Let me explain that, when you just activate your W trynity force will do your first attack stronger than your ad, then the second will be insane too but less than the first instead having the black Cleaver buff, and then your final strike of 3 will be a dealing one, and if you do any critical strike the damage will be increased by infinity edge.

That burst is enough to kill any carry ad or ap squishy champ easyly, and at level 18 this spell have only 3 secs of cooldown.

Pros:
Insane Damage until you get the black cleaver
High Probably of getting feed
Short Burst Cooldown
You could suffer quite damage instead in hammer form

Cons:
Squishy at the begging
Evolutive damage
Quite expensive build at the beggining
Need of knowledge about jayce

So , starting with your cannon form activate your W , then change into hammer and use to the skyes in your opponent, let the magic of the build works, and if the target is not dead yet bat him with your E, you could also activate your lightning field for some good extra damage

Another Combo is just with the cannon, Press W , shot him your 3 attacks, and then use E to create your aceleration gate and shot your Q through.

Why I max first Q ? cause your W increase your damage , but if you dont have enough damage is a waste to start with him, i build it evolutive cause of that

I hope you enjoy defending the tomorrow with that char =)

The core of the build is :
Trynity force
The Black Cleaver
Infinity edge

The other ones could be changed according to your opponents team.