Every site I visited that talk about what Collossus is made out of says that he is made of "organic steel". Organic steel is not adamantium. Diamond is probably the strongest organic material, so I believe her coating is stronger. Also we all know Emma as a munipulative mastermind vixen. Mastermind is the key word. She is extremely sly and intelligent. Personally, I cannot say the same for Collossus. Diamond must be hit at a percise angle and force to be broken.
Does Collossus know where to hit? I'm sure that after the first time being shattered, Emma has researched about the properties of diamond, and know where she shouldn't get hit. She'll endure his regular attack, and evade the ones that are potentially dangerous. You say that Collossus is a "man mountain". Other heros that have been dubbed "Man Mountains" are very very slow, unless they have superspeed, which Collossus does not. Emma can strike and evade him easily.
impressive argument, but emma does not have superspeed, and colossus is extremely nimble for someone his size. im sure emma can and will strike him, but i believe your underestimating how durable his organic steel actually is, and the probability that he most likely will hit her... hard. i know her diamond can take a lot of punishment, but hand to hand in close quarters i just believe he is too strong and durable for her...
When you look at Juggernaut, The Hulk, The Thing, Rhino etc, they all have huge huge frames.
Colossus is just a very very big man who changes into metal. There is no reason why he should be considered a mountain or labelled as "Slow". He's not that much bigger than the X-Men. Bodybuilder size, bit taller maybe. Infact, he's smaller in mass than most bodybuilders.
He just has insane strength and durability.
-AC