With the posture that he had taken to strangle the life out of her, he had full view of her back. As she began to form those spikes of diamond he raised an eyebrow in confusion, maybe he had strangled too much oxygen out of her already. It was a desperate attempt at defending herself, at least he hoped it was. There wasn't much that he could do about blocking from this position, however, there was nothing limiting [i]his[/i] movement. 

Previously she had demonstrated taking time to form her diamonds, so there was more than enough time to move out of the way. It wasn't as though she would fire them off like bullets from a rifle, though they certainly packed the same kind of punch. Showing off the sword gave Shin a wider idea of her abilities than she would have realized. See, with the second demonstration in her projectile she had shown that she was really limited in her ability. As every time she cast her spell, or whatever she would call her ability, she took a few seconds of time to properly utilize it. 

To evade her attack, he raised his hands upwards and kept the chain taut between his arms. As well, he stepped off of her leg to free up both of his legs. Then he put himself to her left side and twisted the chain around itself to put even more pressure on her throat. Standing off to her side and strangling her would leave her without many options. Trying to stand up would warrant more pain, she couldn't use the sword in her right hand, and as the oxygen was slowly lost from her body, she would be left helpless for the next few minutes. 

It wasn't particularly creative of him, no, but he had grown weary of this woman. In the starting moments he had promise for her, demonstrating such a useful ability and attempting to kill him with a swift assault. It was an intriguing ability and he had hoped there would be more show, but he had already put her into certain death scenario, and that meant no more fun. 

Aside from watching her die, but the buildup is often more fun than the conclusion.