Thanks for the Irelia tips Newt.
Jayce Guide:
Build: 90% of the time you start Dorans+pot. If you're against Pantheon, Gragas, Lulu, or Ryze (basically any champion that you can't safely harass without taking some damage in return) starting with a flask+3 pots is a strong start.
My standard build order is Dorans+pot->Tear->brutalizer->boots->Ionia's->Manamune-> and from there it's dependent on the game. GA/BT/BC/Ghostblade/LW/BV are all good choices depending on the game and how well you're doing.
Depending on your play style you may prefer to rush a brutalizer and get a tear on your second back.
Crit is worthless on Jayce because the majority of your damage will be from Q->E'ing people from a far. Armor pen is a much more useful stat for him.
This is the build that I aim to have by the midgame in most games.
Skills: You max Q->W->E->R in that order. Turn quick casting on for every ability for the fastest combos. Since Jayce has 6 abilities with very little casting animation it's pretty much required. Don't level your Ulti until level 16.
Summoners: This is up to your preference, but I run
ignite in roughly 80% of my games. Against champions like Irelia, Yasuo, Jax, Renekton and any other melee who you can dominate you will always want ignite because if it's up you have nothing to fear from them past level 7 as long as you have tied or won your lane before that point.
You should run
teleport if the enemy picks a champion who you know will also pick teleport or if you're not confident in your ability to win the lane. Jax is the only exception if he runs teleport and you run ignite, you should be able to wreck him with ease.
Strategy: In almost every single match up I only last hit creeps that are sub 15% hp and I aim to avoid damaging the creeps what so ever when possible. The rest of the time is spent poking the enemy when they come in to last hit or trying to hit them with non gated ranged form Q's. Your plain Q is your most efficient harassment tool in terms of mana cost and not exhausting your mana pool. You should only use Q->E in ranged form starting at level 5 or if you think you can kill them before that point.
Jayce is vulnerable to jungle ganks in the very early game so you want your lane to get pushed closer to your tower. If the enemy is pushing really hard Q the melee creeps and kill them before they reach the tower, then tank the ranged creeps out of range of your turret and wait for your wave to arrive. Now the lanes frozen under your tower for the next couple of minutes and as long as you kill the melee minions fast enough it's very difficult for the enemy to force your lane to push back out. In doing so you usually get a big advantage over the enemy because they have to give up on some CS, as you comfortably farm near your tower with high AD and a fast attack animation.
Late game your goal is to stay in ranged form for as long as possible in team fights. You really only go into melee form if it's to execute someone or if you catch somebody alone away from their team.
Combo: Once you have your tear your killing combo should start in ranged form. It's Q->E->W->Melee->Q->W (if they're low enough to execute or not dead already and your ranged form is almost off cooldown) then you can hit the melee E, swap to ranged and get an AA+Q to finish them.
