I asked previously about removing the atmosphere of a planet, and one of the responses was through an artificial Coronal Mass Ejection.
On my drive home today, I thought it might be more effective for the DoomFleet to be able to create a low-grade supernova-like effect (By definition, a "standard" nova can only happen on a white dwarf in a binary system), thus hitting almost all of the planets in the system with one event.
I'm not particularly worried about the mechanics of creating such a nova. The DoomFleet has antimatter weaponry and can travel faster than light, so they can manipulate absurd amounts of energy as it is.
What I'm wondering is the effect of the supernova on the planets aside from stripping the atmosphere, and just how strong it would have to be to perform such a removal. Also a curiosity is how protected moons that happen to be behind larger planets, particularly gas giants.
For a sense of absolute power, the DoomFleet doesn't particularly want to expend more energy or time than it has to. So, for example, if it's in our Solar Sysytem, it would be enough to remove the atmosphere of Earth and a terraformed Mars. Maybe not entirely, but enough to make them unlivable.