This guide is a blend of AP and AD, because Kassadin is a Hybrid Assassin. You must mesh together a good amount of attack damage to go along with his melee, and bring along some ability power as to do amazing amounts of damage with your spells. learning that Kassadin can stick around for a long time, which means people will fear you. Rift walking all over them and still having enough mana and health to kill more is a plus in my book. Kassadin is pretty squishy still, if he gets trapped then he will go down quickly, most of the time it feels like i am targeted first when playing kassadin.

Pros:

strong mid to late game
Does a lot of damage quickly
Flashes every 3 seconds (at 18) and does damage on landing
Awesome silencer
Can escape so easy
Great solo mid or laning with some teammates,(blitzcrank ftw)
amazing cooldowns

Cons:

Squishy
Targeted first
harassed easily
takes time to master. but once you do....gg

For runes, Ability power per level, not the other one. and Mana regen, i find that gaining a good amount of mana helps me stay in the fights longer, with riftwalk stacking up. alternatives. not sure i have yet to experiment with others.

kind of weird i know, but kassadin is a melee/mage. which is why i went with the offensive/utilities masteries. you can go utilites masteries build, i just prefer melee mine to be melee heavier seeing as kassadin is in the mix of fighting most the time.

The items above do make what i would call an amazing combination for kassadin, instead of Rod of Ages, you can go Banshees veil, if you are getting hammered by another spell caster. other than that, i like my build and i always go with those items. (btw, you dont have to save up and get the item at one time, go back and get the items necessary to purchase the main item)

Riftwalk in Null sphere, Force pulse(cause he is running away), Rift walk on him, (auto attack) null sphere, force pulse,(you will probably have it again with team mates around you) and use W whenever you get a chance, it will add a stack to your force pulse and do more damage with your auto attack! also the enemy would probably have died. :)

Teamwork-
work together, you will always come out on top, as Kassadin, your force pulse is extremely helpful for running enemies, it slows them SO MUCH, team mates will thank you for it. if one of your buddies is getting attacked, rift walk in and help him escape then rift walk out of there, you can do it with the short cooldown it has.

Skills-
This build is about melee/spell casting. use your auto attack, it has the power to kill, just dont forget about your spells too!

Farming-
As Kassadin with this build, you are able to kill MILLIONS of minions easily at about lvl 9 and pull ahead of the game in money and levels, kill the 3 dog pack and the 4 ghosts when you have a chance, then move up to the two golems.

This guide is my own, i like it, i use it, i win with it.
Kassadin isnt the Easiest character in the game to play, so dont
get upset when you dont do well with him. Take some time to
learn him and how he works, it is worth it.