Any advice for fighting Cass?
Ok, first of all, this matchup is perfectly winnable but I would label it as a 6:4 to cassandra if the player is strong on execution as it basically feels reminiscant of Xianghua, but with more damage potential and better punishers at -16+ at the trade of slightly weaker at lower - and gratifyingly, nothing at -11 ^^. I say reminiscant of Xianghua because the key in my mind to beat both characters is patience, good defence, dominance in the throw game and SRSH.
Offline I've fought five differant Cassandra players, so I'm not an expert on this matchup but fairly well versed as one of these players is nowhere near as good as he would have us believe, one is a longtime sparring partner of mine but is bad with Cass, one I had no problems with at EBO, one is Kalas' and I can beat it because I know him well enough but the other was Keev. Now, vs. Keev the most effective tactic was to take the match slowly, play safefried and catch him out after bating agi's, backsteps AND MOST IMPORTANTLY iFC 3B (God, I hate that move ~_~.) but space control proves much more difficult: Cassandra tops Siegfried at mid range and it can feel a little intimidating trying to keep her out, so the nomal ag:A 33B mixup as you may have gathered is riskier. At mid range, Cassandra can cause you to whiff very easily and if she does, you will eat A LOT of damage if they're on point.
Now as I said, this is a competitive matchup, bujt you have to be prepared to play an ugly match: take it slow, play safe and don't just throw out attacks - unless its a punish, consider how she could beat the attack in that situation and have an option at hand to kill the answer to use as a mixup when you feel it safe to attack: Cassandra players will disrespect frames very readily if they get a read on you, even your most basic pokes need mixups eg:
Cassandra player (and this is quite a common tactic to feel out a player in early rounds) will 33K with no followup to see how you handle it then adjust accordingly. What a lot of people forget is that you have an oppertunity here as well - consider how you handled the situation: did you turtle up, use an iFC move immediately, just duck, move, regular attack, throw? What did you do?
For this example, lets say the Cassandra player knows you will GI 33K A+K on reaction but turtle for fear of 33KK whiff punish oppertunity after a round. Because the opponent is 'just' Siegfried, the Cassie player won't care about the - on hit too much and from earlier reads is lible to multiple 33K's one after another to chew away the health bar. You can make a read on this yourself: you know you took the mixup defensively and know Cass is liable to take this option now, so shut it down: as you eat 33K, commit with a 3B and take a free CH 3B, SCH A+B for your trouble. Now the mixups evolve even further from here, the Cass player can easily reset their tactics from this situation and go into 33K ~ step or 33K into block and this is where throws and 6A come in. You see how all of your answers must have their own counter offensive or counter defensive options must have further options?
I won't go into details onto other string mixups like 3A, 3AK, 3Adelay K as this applies to most of the above example, but I will briefly talk about 4BB:A. 4BB is often used by Cassandra players as the base mixup with throws and 33K/1K at close range and the reason for this is that a GOOD Cassandra can confirm the attack was not blocked and hit the just, making for an i17 53 dmg mid on hit. Now, 4BB by itself is -15 and thus punishable by Siegfried, but not for more than 25 damage, so here I would not recommend punishing to tempt the Cassandra into following through very often: 4BB:CHA is just too painful + KND - the risk/reward is too far in her favour. Most often, take this situation defensively, be prepared to duck and punish the A, but make sure you mount your offense or further defence after confirming no just finish to this string, you won't get free damage, but to keep the pressure on, the Cassandra player has to take bigger risks.
Finally: dun, dun, DUUUUUN! Its time to learn another JUKE! FUN TIMES! Cassandra's biggest launcher is the incredibly linear 33B, should this connect, Cassandra can take close to 90 dmg, but in order to reach this, the first comboing hit is A+B, which on air hit can be JUKED and ends the combo immediately.
There is no formula to beat Cassandra outright, she's far too well designed for that, but her mixups ALWAYS have a certain degree of risk (albeit low) and the biggest step to beating her is truly understanding that and being prepared to make equal risks to counter them. That aside, learn her frame data, find out what you can punish and what leaves her close to 0 as this gives insight into her offence. Maybe even learn a little Cassandra yourself as that's what I do if a character is giving you trouble.
Now, take no offence here, but matchup analysis only really applies at high level and when both players know the matchup well and I'm assuming you and your friend re not playing to that standard. If I had to guess, I'd say what you're losing to is either a situation/s tht you don't understand or even worse IMO, misunderstand as this is what sets one mid level player above another most commonly. If you could record a few matches, I would be able to help you much more directly. For now though, this is the best I can do before work, lol.
Sorry about the wall of text (christ, I'm turning into Dylan, or worse, Signia @_@), but I hope you found it helpful. =)