I am a NA Challenger Kassadin player.I have reached the Title of rank 1 Kassadin NA and have over 1k games on him across all my accounts.
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I stream at Twitch.tv/korasiii

PRACTICE THIS COMBO PLEASE. R + FLASH. USE IT. ABUSE IT. THIS IS BY FAR THE MECHANIC YOU MUST GET DOWN. THIS IS ONE OF THE HARDEST ABLITIES IN THE GAME TO DODGE.

KASSADIN IS BEST FLASH ABUSER IN THE GAME.

R FLASH R FLASH R FLASH.
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KEEP MENTAL CLEAN. PLAYING KASSADIN IS PAIN AND SUFFERING BUT REWARDING IF YOU ARE ABLE TO MASTER HIM. DO NOT PLAY KASSADIN FOR THE FIRST TIME AND EXPECT TO DO WELL. IT WILL TAKE GAMES AND PRACTICE.

Kassadin Pre 6 is probably one of the worst and hardest in game. IT IS ESSTENTIAL TO PLAY CONSISTANT AND NOT TILT.

Try to ward for your jungler incase of invades at 1:15-1:20. Ward their buff or jungle entrances around mid river.

Against Ranged matchups you want to show into lane late and last hit the melee minions with Q and 2 Autos. If the enemy is fast pushing first wave it is acceptable to match the push with autos in order to trim the wave.

Against melee champs you should start W unless they can auto space you (Viego, Yone, Yasuo, GangPlank). Kassadin auto + w is one of the highest damage combos in the game and you should try to abuse this level 1. Do not take extended trades after Auto + W as you lose.

In higher elos enemy mids will freeze the 3rd wave against you as it slow pushes back to them, it is mandatory to crash this wave. Try and communicate with your jungler to shove this wave in, if you are unable to and the wave freezes you must tp back into lane.

Contesting Scuttle is a no for Kassadin unless you are against a favorable matchup early like Fizz, Katarina or Sylas. In most games (Especially lower elos)Try and tell your jungler to path away from enemy jungler so that can safely secure scuttle crab. It is important to keep jungle matchup even.

Kassadin mid game with Lost Chapter + sorc shoes does considerable damage compared to S10 Kassadin who rushed Mana + Health.

Always Save TP for sidelanes if you can. IF YOUR LANE IS SLOWPUSHING BACK TO YOU AFTER A WAVE CRASH DO NOT TP BACK INTO MIDLANE!

As Kassadin, you should look to play like a roamer and push leads for your team if you are ahead. I often compare Kassadin S11 playstyle to Katarina or Talon. Roam heavy playstyle will snowball your jungler and sidelanes to victory.

If you are unable to Roam because you are consistantly shoved in, try to fast clear waves and match enemy mid laner roams with TP or simply following them.

Kassadin is one of the best abuser of Everfrost. Try and Practice the R Q E Everfrost combo. Try to setup Kills with E + Everfrost. The cc from the 2 abilities work wonders.

Fighting for Dragons as Kassadin isn't great. Often in games teams will fight over every dragon. Do not do this if you cannot guarantee a successful secure + Fight. Ping them off.

You will want to sidelane after your bot swaps lane after they take bot tower. Do not lane vs enemy bot midgame as your champion does not 1v2 well. Always keep in mind of TP plays while you are side laning, do not AFK sidelane until 16. You will lose if you do not play macrowise.

Level 11 Kassadin is an important powerspike. If you are ahead of the enemy mid try to rush this level threshold.

Use R Flash to pick squishy + unsuspecting enemies.

Play Like an Assassin. R-Flash.

With 3/4 items you can oneshot most Squishies and safely get out with Hourglass or combo again once abilities come off cooldown.

The importance of Flash in the late game is very essential. As you can close distances you cannot cover.

You can either play fights slow or fast depending on your team comp. Slow fights if you have engagers or fast fights if no one on your team has setup.

level 16. Play for this. It's not as strong as s10 but it is still huge in terms of power. Your R will do ~1k damage per R at full build. Kassadin QWER does around 4k. damage As QWE scales with 0.75 AP ratio.