Maybe. I think the thing with Kiri is that Kouma vastly underestimated Kiri's ability. When Kiri actually causes Kouma to feel pain, Kouma goes a little apeshit because of it. If he underestimated Shiki, then he'd probably die.
It just depends.>>10137571
No, the Kouma nightmare grew progressively weaker the more Shiki fought it. In the beginning, it was faster and stronger than Shiki. It was only when Shiki stopped thinking OH SHIT HE'S GOING TO KILL ME IN ONE HIT did the nightmare become something he could fight.
Then the glasses came off, then Kouma become little pieces.
Shiki stopped thinking that he was an enormous badass before the end, though. He even says to the nightmare that it's "nothing like the real thing." The real thing being Kouma in real life...