first of all hi im jack i have been maining jayce since 2014 so i thought id make a guide on jayce since all the guides here are not very accurate . i have well above 600k on him so i think i know him pretty well . very few people know his potential and are often surprised when they see his damage in game
as he may seem like an easy champion u can very EASILY tell a good jayce from a bad one as u gain huge experience everytime u play jayce

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all these runes might not be cheap and might cost a lot but they are definitely worth it for insta erase with jayce

COST : 7371 IP
RUNE PAGE : 6300 IP
TOTAL : 13671 IP

pros : 1- op kiting
pros : 2- op damage
pros : 3- shreds tanks
pros : 4- assassinates squishy targets
pros : 5- extremely long range
pros : 6- has good movement speed thanks to ghostblade , e and passive

cons : 1- squishy and can be focused
cons : 2- might be useless as tanks are more useful in the current meta
cons : 3- his q , e is hard to aim
cons : 4- bad against ganks
cons : 5- not a very good passive

starting off this is usually how i get first blood : u let enemy push wave at beginning . at level 2 u unlock ur e . use flash and e enemy into turret he will usually use flash and get 1 hit or 2 if u r lucky , which should help u finish him off (train in practice mode as its easier said than done)
tips : 1 - you always want to ward ur area . buy a control ward and ward the little bush next to rift between midlane , toplane
2 - if u arent reaching anything in lane push wave hard and then roam quickly (level 5+ and with boots of speed at least) u should easily get a kill if u r good enough
3 - in mid game hide in the enemy jungle when u know 1 person will come alone when he is close use e , q in ranged and then switch to hammer and go in with all spells and it should be an easy kill ( be careful this needs vision and map awarness)
4 - u get weaker as time goes try to finish fast
5 - eye of the herald is one of the best items make sure u keep it warded or try to get it with your jungler
6 - ur job is similar to assassins which is to assassinate their adc or their midlaner (depends) if they are all clumped together however then stay next to ur team and try to hit as many as u can with q , e combo
7 - ur w passive allows u to get a lot of mana so try to go in hammer form as long as u r sure u wont take any harrass damage . since u have 5% life steal from doran ring , mastery this should also return a good amound of health