Basically, in order to make this a fight, Mags has to have CIS way, way off while everyone else is completely in character. Powerset-wise, there's no reason Mags couldn't trick some of them into breathing in enough adamantium dust to then tear them up inside. In character, Supes wouldn't be using the full spectrum of his vision at the beginning, so as long as he didn't breathe any in, he won't be able to warn the others (I imagine some sort of "super taste" would tip him off).
Mags' 50-ton adamantium sphere + his formidable force fields should keep him safe while he tries out every weird plan we can think of to put his opponents down.
Just because adamantium is "indestructible" doesn't mean that's it's somehow like a lightsaber that can just cut through anything. Bullets bounce off these characters, and they would bounce off if they were adamantium also. Adamantium is only going to make it harder for the team to bust through, but there's some serious invulnerability going on with this group.
Destruction is its language, violence its voice...
Ultron's armor is pretty thin. Denting a plate of metal is a lot easier than denting a meter or more of it. Plus, Mags can repair it with ease. There's no mistaking that his defenses here would be a huge obstacle for the team to overcome.
Right, but adamantium has a hardness that will (almost) always be higher than the hardness of whatever it's up against. That alone isn't enough to cut anything, but it basically means that it can cut anything not truly indestructible provided with the right amount of force... and we happen to know that Magneto can apply that kind of force.
Whether you think that he wins this battle or not, it'd be pretty silly to think he couldn't cut any of the characters in this thread with a chunk of adamantium.
Adamantium has no potential to be sharper than any other substance. The only difference is that it can apparently hold a monomolecular edge indefinitely. In theory any solid substance could be as sharp.. it just wouldn't hold the edge beyond the first swing.