I have noticed something extremely strange about this guide, in that it isn't explicitly stated you need golem buff. I find run out of mana a lot (I like spamming spells on minions :D). Is Cassiopeia a good holder of a blue buff? 

Ofc she is, but if you're spamming her spells vigorously, she can run out of mana even with the buff. I guess it's pretty obvious that casters should carry blue, that's why it wasn't explicitly stated in the guide.

I've got a different problem with Cassiopeia as a character... she feels too squishy to me early on. Unless I set up a trap, I get blown up before I can use my ult in a teamfight. If someone more experienced comes around, could you recommend some sort of intermediate item build order to alleviate that??

Can i use RoA, Rylai and Lisandry at same time? =) 

I would think she need's Athene's with that mana problem and all

cas is hard to play i think

She has massive mana issues, and the best way to overcome it is to keep stacks of Deadly Cadence up by spamming spells. Her stun can be undone by turning around, and her signature Q has more than a half second delay between cast and activation. Her entire skillset greatly encourages aggression, but she has no reliable escape (ult is high cooldown and w slow isn't really that effective without Rylai's). High skillcap champion all right :D

Don't misunderstand me. Cass is an incredible champion. Her RW combo can leave an enemy wallowing in poison while you hurl E at their face. Her damage (sustain and Dot) is incredible, her push is incredible, her gank is incredible, her damage is incredible. Just don't run out of mana 

I have played Cass  a lot  and in s3 i find  your  best best is honestly a  more defensive build wit ha liandry, for example my full build will normally contain a  wota, GA, and Zhonya's   wit hthe Liandry. Loads of defence, still great dmg  with  rabadons and your dots, on top of it all,  when you are  zhonya'd or  ressing  from GA spell vamp continues to tick from your dots. 

What is the perfect team composition for cass?

Teemo, Singed, Gangplank, and Twitch :D Teemo support, Singed top, Twitch jungler, Gangplank AD carry

What about Spellvamp ?

I feel like Cassiopeia is back in the god tier of mages, at least in solo queue. With the increased amount of tanky bruisers mid, picking cassiopeia blind is never a bad choice. I feel like Liandry's Torment with some sustain is a perfect combination for her. Maybe im saying this because i'm not doing very well recently due to lags, but everytime I play her, i feel like the only weak side of Cass is player that plays her.

I have a question though, how does Cassiopeia deal with her sister, Katarina? I feel like it's the only harder matchup for me. her mobility is godlike, and if she turns her back before ulting, i cannot stun her, taking full combo damage. I don't know exact numbers, will Cassiopeia outdamage Katarina in full lv6 duel?

Coming from someone who religiously plays Cassiopeia AND Katarina - I have the same problems going against a good Katarina. She is literally the only person I believe that can counter me when I play Cass. I always manage to find ways to out burst or play other mids, but against an experienced Katarina it can be extremely challenging.

Here are some guidelines I go by when I play against her. Your tower is your friend, stay on it as much as possible. Don't worry about out cs'ing her just try your best to match it (an experienced Kat should will out cs you always.) Only go in to harass once you see her spells are on CD. Katarina's have to use their spells to farm when against someone which such range and dmg as Cass. Watch for her combos too. It's extremely easy to avoid her Q by keeping distance. It's also easy to poke her more than she can you if you notice her Shunpo is on CD, when I see this I start with Miasma instead of Noxious Blast to ensure max dmg in the little amount of time given.

Always keep 3 health potions on you. Always ward. Be in sync with your jungler as much as possible. Remember than your ult DOES NOT interrupt her ult unlike 98% of your other match ups. Try to catch her off guard. Your best chance at killing her w/o help from a teammate is to poke her from anywhere to 50 - 70% health remaining w/o allowing her to trade dmg with you, then wait for the opportunity when she's facing you and ult her. Afterwards be ready with your combo and never do this w/o an ignite.

Try to think of this match up as Annie against Cass. IMO Cass crushes Annie, EXCEPT when she catches you off guard and combos you. Do this to Kat with everything in your kit. It can be annoying to have to wait for ignite to be up anytime you want to kill her but it honestly does the trick. I also understand it can be extremely frustrating to have to stay on your tower. Don't let her push you to your tower when you're low. I can't count the times I enjoyed tower diving people when they think they're safe on their tower with low health as Kat. Hope this helped. Sorry for the run ons it's late. Cheers.

EDIT: Did I mention that Chogath is a jerk? I hate laning against Chogath's. lol

How do you feel about Liandry's Torment?  I feel like it would be stupid awesome on Cass.