I think... the issue here is that... if Corban's defence here falters, it might indeed spell his doom. He made absolutely no attempt to actually dodge the beam, so if it hits, it hits him dead on, exactly at the vitals Myron was aiming for. It can pierce his diamond dome with ease, and has 100ft of girth behind it to spear him in place until he's done with life. I doubt he'd be able to concentrate on erecting barriers when his heart, lungs, and spine have all been reduced to ash.

Given the situation, if I was in Divinity's shoes, I'd also argue every single little speck of information I could in order to save myself the loss.