Does Fox-Fire kill everything with 4 might or lower individually, or any number of units whose combined might totals 4 or less?

Ruling: Fox-Fire kills any number of units whose combined might is 4 or less (e.g., a 2 might and a 2 might, or a 3 might and a 1 might). You must choose and declare targets when playing the spell, before opponents can react.

Sequence:

• Choose targets when playing the spell (units whose combined might is 4 or less at a single battlefield)
• Opponent can react knowing your targets
• Spell resolves and kills the targeted units (if still legal targets)

Nuances:

• All targeted units must be at the same battlefield ("at a battlefield" means only one battlefield)
• If an opponent buffs a targeted unit in response, making the combined might exceed 4, the targets become illegal and the spell has no effect
• This works similarly to other targeted removal like Gust - you can buff units out of range as counterplay