Matchup Ranking: Well I haven't played CSEX yet, but if its anything like the CS2 MU (which it looks to be that way) than its 50-50.
Overview: So this MU is a lot like the lambda vs Ragna MU IMO. The main difference is that Mu has projectiles. DO NOT THINK SHE CAN OUT ZONE YOU! Because she can't, do not let her set up steins wherever the hell she wants, you will regret it later on. If she starts to throw out steins all willy nilly just 6D or 2D (dash 6D/2D works as well) and CH her. I fine that this MU is one of the more stressful and hard to handle MU's that Lambda has just because Mu CAN force your hand. On the same token, Lambda can force Mu to do things she doesn't want to do as well. What I'm getting at is that you must be in control of this match or you will lose horribly (that of course goes for Mu as well). 236D Oki is very strong vs Mu because (as far as I know) she can't really do to much about it, plus as I said earlier, the person in control of the match will win, so naturally 236D oki goes well with being in control.
General Strategy: ZONE HER! Do not get into the corner against her, its a freaking nightmare. Its hard to summarize this MU just because of how diverse and awkward the situations you may find yourself in can be. You want to zone her, but then she spams steins, so you rush her down, then she rushes you down. Its so weird, you just have to find the nice middle ground in which shes trying to stop you from controlling the match rather than you trying to stop her from determining what happens next. Overall this MU is just a solid 50-50 MU between two vs close characters on the tier list.
Good Tools For The Match Up: 6D, 2D, 236D (oki), 5/2A. 6D and 2D are good AA tools used against Mu that work surprisingly well. I have already went over 236D oki a lot in the above. 5/2A keeps her locked down, which if your a lambda main you know how it feels to be locked down.
Things To Look Out For: 6C (OMFGWTFBBQ DONT GET HIT BY 6C), exploding steins, 2B, and last and certainly not least... 2C!!! 6C... How to describe 6C... Its like being hit by a truck and then eating a ragna blood kain combo to the face (i'm exaggerating calm down). But seriously don't get his by a CH 6C, shit hurts so much. Exploding steins, Oh yes these should not be hitting you, but when they do they hurt like a bitch. This comes back to when I said not to let her place steins wherever the fuck she wants to, because these exploding bastards will bite you in the ass. 2B is what every Mu player uses to approach/poke. Its just an all around good move for her. 2C oh... my... god... 2C. That AA is soooooo godlike, it makes me want to cry my self to sleep. AND IT LEADS TO BIG DAMAGE!!!
Conclusion: Zone her. I didn't repeat anything (at least I don't think I did) of what Nemesis said because he covered most of it I just wanted to give my 2 cents about it and from the Lambda side of it. Now go read his post if you haven't already!
Last thing before I'm off, if you guys have any specific questions or situations just bring them up. As I said earlier its hard to summarize this MU just because of how awkward some of the situations can get (for both characters).
