Jayce damage is off the charts right now. After trying many rune pages and builds i found this one having the most potential. His damage was always high since his release back in season 2 but the new rune system favors him a lot. He is great for low and high elo games and can be played to either stomp your lane early or safely farm and kill every carry with a single combo in late game. He is also a great split pusher since he obliterates minion waves and turrets thanks to his high damage potential.

Pros:
+ TONS of damage.
+ His E has multiple uses (saving himself, throwing enemy into your team, High damage as a last hit when fighting).
+ He can buly his lane easily in early game.
+ He can kill carries with one combo.

Cons:
- Squishy
- You have to train his Q+E skillshot as it is very important to never miss it (it does TONS of damage).
- All his skills must be used in different order according to each fight in order not to be a sitting duck. There are fights that you must stay in Cannon Stance a bit longer and other ones that you must go All In with Hammer Stance. That NEEDS practise or you may find yourself in a team fight changing into Hammer Stance and realising you can not dive in so you just lose important seconds that you can not do anything.

I wont overanalyse this because i think i do not need to. I tried lots of runes and i found these to do the most early game damage. I also like the Domination path but i am not really satisfied with its 2 last choices so i kept is as the secondary path.

I also wont post a long detailed explanation about the items because the title of every build explains it all:
-If you are winning your lane follow the 1st item build.
-If you are tied in lane and it is a farm fest but the enemy team has not ways to kill you a lot then again follow the 1st item build.
-If you are losing your lane but the enemy team has not ways to kill you a lot then again follow the 1st item build.
-If the enemy team has many ways to kill you and dives on you a lot follow the 2nd item build no matter if you win or lose lane.
-If you are against an enemy who does heavy AP damage to you follow the 3rd item build. To clarify this some enemies, like Cho Gath for exaple, do scale with AP but they should not be able to ever come close to you so a Mercury's Treads will be all you need. No need for Maw of Malmortious in this match up. But some times you may face a Vladimir top or anyone who feels like to play something like Syndra (yes these picks exist sometimes) so you will need the extra MR of the 3rd item build. Even against a very offensive Maokai you will need the 3rd item build.

Jayce has many mechanics and not only one proper skill order. It would take a wall of text to explain all these. Istead just watch this video witch explains them all.

The video is not mine. Congrats to the guy who made it.
Play Video

Lane phase is totally different according to the enemy champion. In the "Threats to Jayce" section (above) i explain what you will have to do according to every champion you may face.

Some general rules:
- Do NOT push the lane if you are not sure you have an advantage. You are a squishy champion and if the enemy dives you, you must be close to your turret to escape.
- If you have a clear advantage and decide to push the lane always have a ward up. YOU ARE EASY TO KILL. You have to remember this. If you get ganged with the lane pushed towards your enemy and you did not see the gang coming with wards you will probably die. Except if the enemies fail their CC/skillshots.
- BULLY early game. At level 3 your Q+E combo is devastating with these runes. Two of these combos will give you advantage. You will get their potions, you may zone them and if they are not tanky they will HAVE TO go back or stay and die with one more combo. Some times your advantage after the successfull Q+E will be so clear that you may instantly R+Q+Hit+E them. This will leave them with so low health that they will be forced to recall. If they are not tanky they will just die. You just have to know against which champions you are safe to dive (read the "Threats to Jayce" section above CAREFULLY).

If your aim is good you can decide a teamfight before it even starts. A solid Q+E on their ADC gets 40-70% of its HP (according to how fed you are and they are). Supports may heal them but you can Poke again and again esspecially if both teams are reluctant to start a fight and try to find the best positioning.

The same applies to squishy Supports, Mid Laners and Junglers. One Q+E takes out much of their health and forces them to have a disadvantage.

So just POKE. A lot.

When the team fight starts you will have an urge to dive in and create havoc because your damage output is huge. DO NOT DO IT. You will just die. You can only do this if your find a squishy target in a wrong position and you are sure that you will it instantly. Keep your Cannon Form and stay behind your initiator. Go in with Q+E or with hit+W depending on what the targets are and when things get more clear you decide your next move:
-If you are not focused and your team is winning the fight quickly use R and Q+hit the target that you will believe is the most appropriate one. Most of the times these are squishy ones but some times your team has to take out a Darius for example and you must help in this.
-If your team is winning but you are focused make the best out of it. If you are sure you will die use R+Q+hit+E on the closest and squishiest target possible so your team will clear them out. Dont Run. You have no escapes except of FLASH. If you are focused but you can escape with FLASH do it and when your enemies refocus on your other team members return and destroy them as your cooldowns are all up again.
- If your team is losing you can either turn the tide of the fight by killing the enemy most important carry with one combo or just realise that the fight is lost no matter what you do and try to save yourself.