Samurai
Adventurer
Remember that AOE spells like Scorching Burst have one die roll for damage. A Level 12 wizard should have a +2 or +3 implement, 1d6 damage + 5 INT, so that's 8 or 9 minimum (good incentive to have the +3 implement if 9 is the threshold). With the +3 implement, that's +14 (=+5 INT + 3 implement + 6 half level) vs Reflex, given at 22, requires 8+ to hit (65% chance). So the wizard will actually kill 2/3 of them.
If the threshold is 11 instead, then it depends on that 1d6 roll, so the wizard now has about a 66% chance of killing 66% and a 33% of just bloodying 66%. Assuming I can do math, we get 45% dead on average (assuming the same number in the burst).
So I guess it's up to what you think is good.
I try hard not to clump my minions such that more than 2 can be hit by a burst 1 attack, but this is really hard in dungeons (KoTS). I've found them to be rather ineffective when they don't have ranged attacks (Goblins)... the Kobolds were at least a threat with Javelins.
IMO, 45-66% dead in 1 use of a 1st level at will power is plenty. As I said before, if you really want to take out a mob of minions, use an Encounter or even a Daily power, as those will almost definitely kill them all, no problem. I want Minions to have a decent chance of surviving an at will power, such that using something stronger is no longer a foolish waste of resources, but a tactically sound idea!