Ah my mistake, I thought it could be CC'd.
With regard to the "flaw", I'm not sure if you mean it was badly balanced, but the math is spot on. 750 * .30 is 225, * 1.75 = 393. The damage is ... upthere, but it's spot on working as intended.
I was thinking about this as I started my latest playthrough, and that Kraken that attacks the ship ... if we get to a point that monsters have deeeeeep health pools to prolong the fight, then you're absolutely right, spells like this undo that. Right now it feels balanced because everyone's health is relatively low, but this does keep the game feeling like a game of "Rocket Tag" (e.g. D&D post level 12). The first one to lose armor, loses. If they want to make health pools meaningful, even by tripling their current values, this combo just becomes that much stronger.
I will say with regard to the "ease" of the fight, I've definitely crushed it every attempt after the first, but I suspect people forget the tension in their first playthrough. Before I knew how best to build my characters, before I knew how to shop for my gear and stack stats, before I knew a worm would even show up, this was a challenging fight for me. And then boom, a void worm opens up for a 70+ point aoe on my party in the second round. I thought I was done for until the magisters started helping. It felt epic and awesome. I think it's just right. Adding it at the end may pace the fight better, but a little chaos and working with your enemy while still fighting them? That's a difficult and fun moment to engineer in any game.