(Warning: do not try this at home-system)
Hello humans, we need advice.
Let's say we have just successfully defeated and killed a Horror From Beyond Reason who happened to be multiple times the size of your planet (what's it called now? Earth?), and wanted to barbecue it (no, we don't want to eat it, why would you ask?! We just... want to make sure that it doesn't come back as a Zombie Horror From Beyond Reason. That sounds unpleasant.)
The first idea that comes to mind is to throw it in our star, but we would rather keep it as a trophy to warn potential enemy civilization not to intergalactically f*ck with us, so the star is a no-go (it would liquefy it! Plus, we don't really know what would happen and don't want to risk losing our lovely star to an Eldritch Explosion.)
Here comes the question:
How could we make a gas giant (suppose it's like your... Jupiter, is it?) flammable?
There are lots of elements in there (mainly helium, argon, carbon, nitrogen and, most importantly, oxygen, though not in its elemental form).
What can we do/change (using the fewest resources and energy possible) to be able to ignite a planetary-scale fire on Jupit- I mean, the planet we have that resembles your Jupiter?