Mind Hax wouldn't work simply because the Leviathan themselves don't have a mind of their own, but a Hive Mind that is centralized in a spaceshift placed deep into space. To affect a Leviathan's mind he would need to reach the spaceshift with it, which would require a Universal range. And even if he was able to reach the mind in the spaceshift, there is no garantee that he would be able to Mind Hax it since it is most likely artificial in nature.
Can the spaceship be that far away? IIRC we only see them attack through a portal, no? Also, their hive mind is type 2, not type 1, so doesn't the individual unit have an individual mind as well?
Oxygen wouldn't be that much of a problem, considering their body is mostly cybernetic and they live in space with no issues.
They don't have self-sustenance, so I think it might.
The hot flames, instead, won't be too problematic since the outer shell of the Leviathans is able to tank the heat of Iron Man's lasers, which were able to melt throught an army of drones like butter. I guess it's not listed in the page because it's his outer shell that resists high temperatures, but I think it should be added honestly.
Thing is, this isn't about burning a hole into it or vaporizing some armor via a targeted beam. This is about just turning the environment too hot. The point is Beatrice doesn't need to melt their armor. She just needs to make it so hot in the armor that they die of dehydration and that's what she can do.
She doesn't need to heat them to a gazillion degrees. She just needs to heat them enough for the water to evaporate.
The Leviathans and the Chitauri are able to survive for prolonged period of time in the deep space, they basically live in it, and since being in space for long times is almost equivalent to Absolute Zero I don't think that would one shot them.
Surviving in deep space for long periods is not even close to absolute zero. Remember that every space station can do that. You just need a little heat from the inside to keep warm, as, while space is cold, it is terrible at draining heat. One big problem for spaceships is actually not to overheat, rather than not becoming too cold.
To that comes that absolute zero has quantum effects which lesser levels of cold doesn't.
So yeah, surviving in space is no feat of protection against that at all.
Yeah, the Leviathans can't kill him because of his Immortality and Regeneration, but he can't put down the Leviathans neither since the only dura neg hax I could find in his profile is Sleep Manipulation, which requires to affect the Mind in some capacity, something hard to do when the Mind is centralized deep into space.
The mind stuff is not true. Fushi uses a potent sleeping drug for that purposes, which he can disperse through the air. So it puts things to sleep via their biology.
That aside, it doesn't matter. Fushi could continue fighting them until they die of old age and win that way.
Not that it would be necessary, if he can simply turn into a flea and get inside the creature to then fill it with tons of molten iron form the inside. Or similar things, like chewing it up inside out, like a parasite. Or disable the tech.
Fushi is also constantly growing and learning to reproduce new things. After a while of fighting the thing it would be able to reproduce its tech.
Basically, it will take a while, but Fushi will without a doubt win eventually.
If Fushi is able to beat Glaedr than I think him and the Leviathans can share the position, imho.
Fushi can't beat Gladr, since Glaedr would just mind hax him.