Your assuming it was at it's highest? There is no concrete evidence to support this, and it seems rather ridiculous.
If orochimaru stays in the mist constantly for a period of time(not long) the mist will melt his body down. Theres also boil release combination. Oh wow I forgot snakes can sense body temperatures but how long of a process would that take before Mei's lands a deadly blow within the mist? Saying the mist is 100% useless is fanfiction as it gives Mei time and defense in regards to countering incoming attacks or planning a attack in advance.
Something your not taking into consideration here.
Orochimaru saids the bodys fluids from his giant snake form vaporize when exposed to air. That being said won't the acid mist basically get rid of all his body fluid? Not too mention the vaporized portion of the fluid would collide with the acid mist right?
Oru has wind style to blow the mist away. Why/how would the acid mist get rid of White snake poison? If it couldnt get rid of Zetsu spores why/how would it get rid of Invisible poison in the air (similar to spores). Think about it,,,
You are assuming just as much as i am with no proof that she can raise it. Thats the same as saying Minato can use sage mode or summon Ma and Pa isnt it? The standard level she could use should be the level she used on sasuke. He was her enemy and she wanted him dead, why wouldnt she make it the highest?????
Oru would sense any and all deadly blows Via snakes and Can Shed his skin if It is too damaged. Manda Rapes Hydra Is over kill, and other basic snake summon combined with Oru's Underground Snake head sneak attack is simply too much! Oru has too many options I literally cannot pick which one is the easiest. LMAO
And Don't get it twisted, i like Mei just as much as the next dude (I defended her before she became "Popular") and ppl finally did there research on her.