Seems a little high, so not sure if I am reading it correctly.
Gaunt Summoner, Searing Warpfire:
So the max dice I could roll is 6, the most that could hit is 6, the max damage each could do is 3, for a grand total of 18 total damage. My quick calculations of the average damage come out to four and two thirds, so I suppose as long as it hits someone with heavy armor it is not quite a one shot kill on average.
Armor is a big factor, as someone like the Stormcast could shrug off the entire 'hit' half of the time.
Still, D6 attack dice is huge, as it forces the heroes to drown in saves. By my (very Math-hammery) calculations the attack yields:
2.91 Wounds to a Hero with a 6+ save
2.32 Wounds to a Hero with a 5+ save
1.75 Wounds to a Hero with a 4+ save
1.16 Wounds to a Hero with a 3+ save
so even if the Summoner rolls average, the heroes can't soak more than one of these.
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It's worth remembering as well that the Summoner doesn't always get to use this attack that often based on his behaviour chart, and by the time you face him everyone will probably have a few treasures and skills which may help out too if they boost defence or cancel wounds.