if in the desert, or ripples on water yes! but regularly no! plus he could just use shadow clones to confuse you while invisible.. and dont forget about the earth fire and wind techs! Muu is just an all around hard ninja to defeat! but you will never admitt that unless his/her name is (naruto, sasuke, minato, itachi) or other main main stream ninjas..
overall the weakest and lamest excuse ppl use to defeat muu is "genjutsu", when it i A FACT that not all genjutsu are the same and will work THE EXACT SAME WAY! only noobs say,"itachi is the best gen user ever, he could surely beat him". when itachi and the mizu are nothing alike or do not fight silimilar in anyway, shape, or form! itachi may possibly be the best gen user, but that doesnt mean his gen will work the same way jmans song gen works or the mizus clam jutsu works!
some of you make no sense (not you anub and some of the others) i swear!!! U_U
" if in the desert, or ripples on water yes! " - Itachi has spent years within Akatsuki. Safe to say he has jutsus that can create water or desert. Also I will like to add that you can see footprints on grass as well. Sharingan will be able to pick up the grass moving. The only way Muu would deflect properly is by using shadow clones.
Another point I want to make: If Muu doesn't know about Itachi's abilites, he will look him straight in the eyes. That should mean (by definition) Muu getting trapped in a genjutsu. Unless you are suggesting that Itachi needs to actually see him for the genjutsu to work, which I won't debate, it's a matter of perspective with (as far as I know) no proof whatsoever.
"Muu is just an all around hard ninja to defeat" - YES. Couldn't agree more with this one. Muu is one of the most complete ninja out there, but he is still not that high in my opinion. I already specified why. But I would definitely have Muu in the top20 and that's high, especially with all the new shinobi we're seeing now.
"but you will never admitt that unless his/her name is (naruto, sasuke, minato, itachi) or other main main stream ninjas.." - I'm not into mainstream at all. My opinions are not even remotely based on that. I was a fan of Itachi's powers ever since he showed his red eyes in Konoha when he wasn't mainstream. One of my fav chars is Neji, who isn't mainstream. And I cannot stand Naruto as a character, but I will admit that he is strong. And I will openly admit any mistakes I've made if new manga chapters
prove me wrong. Until then, I have not seen enough from Muu to surpass the best 5-10 shinobi.
With regards to your genjutsu comments. It's true. If you were to ask me who would win between Itachi and Jirayia sage mode in a genjutsu battle, I would say that I need more details like where the fight takes place, are there any obstacles between them, how far apart are they, etc. It's never as simple as Itachi =10, the rest are between 1-9 , so that means Itachi ownz the world. I won't count Itachi's genjutsu as his only weapon and worst case scenario, if Itachi was smart (and he is) he would just create a circle of amaterasu around him and (1) escape underground and devise a surprise attack or (2) use susano'o and swing for the fences. If Muu gets close enough he will be caught eventually.
In the other's genjutsu theory defense, I will say that they might want to underline that Muu's weakness is genjutsu. And genjutsu, even if split in categories, works the same way: it "traps" your brain in an illusion. So in other words, the best way to put it is : Muu's weakness is genjutsu defense. It might be more accurate.