The comparison with grenades was to demonstrate that much power shows clear intent to kill, not to say that it always happens. The nature of the adjective ladder is that +2 to an effect often represents an order of magnitude greater in effect. That's why swords and grenades are separated by that amount. The point is that weapon: 6 effects are dangerously strong, and any wizard capable of using such should be fully aware of the likely consequences of using them. It strikes me as extremely unrealistic, and counter to the very nature of the setting, for a wizard to casually use those kinds of attacks on pure mortals, without some expectation of death. I could easily see reason in letting a mortal or two survive such a hit if the rolled well enough on their athletics to represent the fact it grazed them, but in the give scenario, that would pretty much not happen. At best, the player would expect them to have an athletics of three, so assuming he meets the control roll, at least one of the four guys he roasts is eating 12+ stress. There is no way a player should be able to do that and escape the narrative consequences of breaking a law of magic. In the extreme circumstance of the PC having evidence of his magic being self-defense, and justifying the killing on those grounds, you might get out of lawbreaker, but at the very least, mercenaries would be dead, and the wardens would want you that way.