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.
I don't really see how you've come to that conclusion, considering the nature of Divinity's defence it would still be relatively easy to re-direct the attack away from vitals, not to mention you can't expect the full force of the attack to force its way through the barrier erected, even if it is weaker in terms of preps, one single prep does not provide you with an instant win upon landing the attack, your both now at the same tier level so all magic is roughly equal, the barrier may fail but the amount of power that then impacts Divinity will still only incur one preps worth of damage. That's how extremely powerful magic in confrontation is balanced.
In regards to Divinity; Figure of speech, but you have reached an impasse over this issue, all I meant. A prep is basically anything which can reasonably be construed as either the preliminary action before or the magical action required to carry out an attack or defence in the future, this doesn't necessarily have to be magical, though most people prefer to work with magic because it is easier. It usually has to be visible, though that depends on power level and the capacity of the fighters, and it cannot be entirely vague, though it's not exactly a requirement to explicitly state exactly what your future intent is, it has to be at least guessable by both writer and character, that would be my interpretation.
That being said, if your claim is that the barrier is prepped, it's up to Innue to judge after all I suppose. I can't personally see where the barrier was prepped for this action, if it's obvious and I've just missed it maybe I can provide an outsiders perspective though. This is where the very specific nature of Dazsos sheet is important, the effects of his spells are explicitly stated so he really needs to do no more than name the rune and mention it being activated, or placed, or whatever Myron does. However, your character's abilities are more vague and require more explanation in IC, if you intended to prep a barrier in case Myron had a counter-attack you kind of needed to state that intent, especially as it's the sort of thing that needs rationalisation in my opinion. Unless your character is particularly defensively minded, or fearful, pre-preparing a defence would require some knowledge of the impending attack, considering the situation I'm not sure that exists. Also, just looking at the sheer volumes of actions undertaken in a short period, I'm not really sure that Corban ever had time to start preparing a defence.
Anyway I'm going to bed, all I can say is the best course of action is a compromise over damage, bearing in mind that T1 Eden dictates that some damage should be incurred from a prepped attack, but considering the nature of magical opponents the damage shouldn't be life-threatening but a mild impediment, perhaps a semi-disabled arm or other similar wound.