Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Kassadin is very squishy and weak mid game, if solo you should just stay back and harras. be careful about missing out of action players because your escape ability ain't that good.
At this point you will have your Rift Walk witch is great for entering and leaving combat. You can also stack Rift Walk up in a bush for awesome burst when entering and stacks up your Force Pulse. Even tho Rift Walk costs a lot mana at this level so pay attention to that you can also cast other spells when you're in there.
Kassadin will be very powerful if played correctly by this time of the game. Still squishy but a awesome burster and can easy kill someone with a couple spells. When engaging team fights come in in the back and don't get bursted down in the start.
My overall picture of Kassadin
Kassadin is a very powerful caster if not the best in the game. Late game try to get the most buffs you can. Ups/downs Ups: Sick burst and good map control, also a good escaper with high mobility. Downs: Squishy and weak early game. Easy to get to over confident and run into a 3v1 or such thinking that you can win. Sometimes it works but not to often. Remember to keep reports if someone leaves their lane or such. Be wise and most of all destroy the other team and have fun ; D