He takes a **** tonne of practice >_< GENERALLY I rely on other initiates of course and jump enemy casters, morgana, soraka anyone else who annoys the **** out of whole teams.

The archangel's with Kassadin is more about the mana pool bonus cause trying to farm golem is just a ***** if your team can't be bothered helping. Takes away a lot of your time you could spend helping your team mates. It also helps a deal boosting your ability power that +3% yet again gets boosted by Zhonya's its a good little bonus. If I was going to implement a Rylai's I'd be getting it earlier dropping will of ancients and bump the buying of the other items (this would be a really good choice of building with Rylai's when your team is AD DPS and tanks)

The real problem is that a late game Rylai losts a lot of its utility. And you'll most probably have to get Abyssal or Void staff as a last item depending on enemy's MR and heroes (57MR is really nice). I would replace the archangel with a Rylai and farm the golems, but it may not be a good idea (i don't know why but i don't really like archangel).

The rest of your comment is of course true : i make Mejai only with Karthus, Morgana and Soraka for my part (the first easily gets kills and assists, i play the second well enough to get stacks and keep them, and the third can get assists on all the kills of the game with that damn ulti).

Cleanse part is true too, but you have to make sure you have it before any attack. if you manage to survive without any defensive item, it will always be better for your dps output, but it needs a really good experience and skill with the hero (not to mention map awareness, warding, etc). I don't have these with Kassa yet, maybe i should play him for a while :p

I also find death-fire to be underrated and with Kassadin's lack of a 4th attacking skill it really does help. The Rylai's would work of course but I find the extra health isn't that needed. Kassadin plays more as an anti-mage and scores more assists than kills in the long run (I don't like Mejai's for the reason that it is unreliable) back to the Rylai's you've got cleanse, riftwalk and from deathfire + archangel's your mana (pool and regen) is pretty decent so if you get CC'd or are in a tight spot cleanse -> riftwalk (riftwalk can get you out of the middle of any DPS so I don't see much point in the health boost of Rylai's) however I did look past its snare component it would be a good substitute for another item like the Aby Scepter.

Your build is a bit weak. I mean you don't have any HP, nor in runes or in items. A Rylai would be great early game so Null sphere an Riftwalk could snare too, and the main point : that gives you a nice HP pool which will save your *** many times. Strange to see a Kassadin without Mejai too, but i just can agree that swowball items are often a bad choice.

Deathfire grasp is a very good choice and is often underated.

I wait for your explanations to make a decent comment :p