Runes
Electro- matchup that has hard time dealing with you or someone that you have to trade with
Fleet- most matchups that are hard, mainly ad and champs that you have a hard time trading with

Build-
RoA - always unless you go 1v9 Chad kassadin

Skipping roa in to archangelz - 1v9 kass, usually if enemy team is mainly squishy and your teamates are so bad so you have to 1v9 before you get 16
Sumns -
Tp - always gives split pushing and makes you less punishable for your trash wave management

Ignite - only go if paired with electro and 1v9 chad kassadin

Early game with each playstyle
Passive kassadin - farm and trade with q e, if they walk up to close w, when hitting 6, you can do short trades with q r e w, after archangels and lvl 11 try to make a play at an objective or else your team will afk and flame you for not doing **** all game while your enemy mid laners is perma roaming or just mute your teamates and farm till 16. If your team does not feed enough and you ignored playing team, you should be lvl 16 before or at 30 mins. Wait till your team engages and pop their backline when they waste their abilities, now group with the team as you legit one shot every one that has under 3k hp. If you dont r into a fight first you are guaranteed to win the game unless they cc lock you.

Chad kassadin -
If you did go this build and are actually going into a squishy matchup such as veigar or lux, poke with your q shen ever it is up. If you manage to get them low, at level 2 cheese them with a quick q - flash - auto - w - ignite. They should be dead if not they probably used their flash meaning they are super low and can get easily dive by jg unless your jg is an ape. If you dont think you can cheese them, or you dont reach lvl 2 bc you let them push into you for some reason wait till 3 and do the exact same combo but add your e as it does the most dmg in your 3 abilities. The lvl 3 all in is the most guaranteed unless you go against a champ that can save themselves aka blood headass (cringamir). If you do pull it off, farm till you get 1300 gold bc the enemy laners probably has tp bc their too puss to go ignite like chad kass. When you get lost chapter, just keep playing aggressive and when you get lichbane ( around lvl 9) roam and one shot the e-girl support and her adc that bought her from egirl.gg. at this point the enemy mid has left the game or you keep nuking their team. Only way you lose is if you choose chad kass into a tank that can one shot you (sett) or legit the only squishy they have are their mid and adc.

Synergies with kassadin -
Top - ornn bc that chad man gives you the upgrades late with makes you more cancer, usually put the upgrade on zhanyas the stats are too op
Jg - zac, he can engage, gank well, and wombo combo makes it feel so satisfying with his e and your r e combo
Mid - ur self
Adc - jinx , aphelios bc those champs can actually carry you till 16
Sup - engage support (naut pref) bc you need someone to engage or else you are stuck sitting in mid waiting for someone to engage but your support is soraka
Yuumi - if you have yuumi, most likely the cat will always stick to you, but a spirit visage bc you will absolutely not die and team will lose mental

Bans - perma ban talon, even if you lane well against him, he perma shoves and roams and your team doesn't listen to pings so they ff15

How to win - tell your team your gameplan as they will think you will come to a scuttle fight but you really cant, play passive and wait for your jg or enemy to make mistakes, your only way of getting solo kills really is to all in of poke with your abilities once you get roA.
Dont lose mental and tell your team to back off, if your level 15 and drag is about to come up, run to a side lane and farm for it, at this point getting aced means gg. Also dont be a ***** when 16. You've been a ***** all game, why would you end like that, be a chad and carry your team that stalled out the game for you expecting you to carry. Also when roaming level 11, try to get the objectives, you do insane dmg if you have lich