If I move to an open battlefield with Inferna, does she get her assault boost?

No, Inferna does not gain the might boost from Assault 2 when moving to an open (unoccupied) battlefield.

According to Rule 807.1.c and multiple RiftJudge FAQs (e.g., #863, #9128):

• Assault is a passive ability that is only active when a unit has the "Attacker" designation.
• The "Attacker" designation is only granted during Combat, which occurs only when units controlled by opposing players are present at the same battlefield.
• Moving a unit to an open battlefield initiates a non-combat Showdown. Since no opposing unit is present to defend, your unit does not receive the "Attacker" designation, and the Assault ability remains inactive.

Summary: Because Inferna is not an attacker in a non-combat Showdown, she does not qualify for the Assault 2 bonus.