Combust

• WWizards of the Coast2010-08-15
  Official Ruling
  If the targeted creature is an illegal target by the time Combust would resolve, Combust won’t resolve for having an illegal target.
• WWizards of the Coast2010-08-15
  Official Ruling
  Combust can be targeted by spells and abilities that try to counter it (such as Cancel). Those spells and abilities will resolve, but the part of their effect that would counter Combust won’t do anything. Any other effects those spells and abilities have will work as normal.
• WWizards of the Coast2010-08-15
  Official Ruling
  Spells that create prevention effects affecting the targeted creature can still be cast, and abilities that create prevention effects affecting the targeted creature can still be activated. However, damage prevention shields (including those created before Combust was cast) don’t have any effect on the damage dealt by Combust. If such a prevention effect has an additional effect, the additional effect will still work (if possible).
• WWizards of the Coast2010-08-15
  Official Ruling
  If a static ability would prevent damage from being dealt to the targeted creature, it fails to prevent the damage dealt by Combust. If that ability has an additional effect that doesn’t depend on the amount of damage prevented, that additional effect will still work. It’s applied just once as Combust resolves.
• WWizards of the Coast2010-08-15
  Official Ruling
  If a creature is dealt lethal damage by Combust, it can still regenerate. If it does, the damage marked on it will be removed from it.
• WWizards of the Coast2010-08-15
  Official Ruling
  Effects that replace or redirect damage without using the word “prevent” aren’t affected by Combust; they’ll work as normal.

Rulings via Scryfall