my opponent play move 1 inferna in an open battlefield. i react with bellows kkilling it, does inferna get +2 assault?

No — Inferna doesn't get the +2. Assault is a passive ability that's only active while the unit has the Attacker designation, and that designation is only granted during Combat, which requires units controlled by opposing players at the same battlefield (Rule 807.1.c–d).

Moving or playing a unit to an open, unoccupied battlefield creates a non-combat Showdown, so Inferna is never an attacker there. She stays at her printed 1 Might, which means Bellows Breath's 1 damage is lethal and she dies.

Worth noting the flip side: if that battlefield had been occupied by one of your units, Combat would start, Inferna would be a 3-Might attacker, and a single point of damage wouldn't have killed her.