You put a high damage attack on all enemy types to see how it would fare. It was Knockout Blow, of course even at minion level damage it's going to hurt. But those attacks come less frequently in regular attack chains and yes --although more powerful critters can one-shot player squishies-- it's normal and to be expected that you might get randomly taken down by a powerful hit. Work on your defense for weaker critters attacks, and make sure the more powerful ones are focused on someone else. This was normal and expected in the old days, and meant to encourage teaming. Sorry if your defender isn't soloing AVs. Even if some people can, that shouldn't be the norm.