If I attack an open battlefield, do I get an Assault 2 trigger?

No, you do not receive the Assault bonus when attacking an open (unoccupied) battlefield.

According to the rules and FAQs regarding Assault:

• Assault is not a trigger: Assault is a passive ability, not a triggered ability. It is either "on" or "off" based on the unit's status.
• Requires Combat: Assault is functionally short for "While I am an attacker, I have +X [M]." A unit only gains the "Attacker" designation during Combat.
• Open Battlefields are not Combat: Moving a unit to an open or unoccupied battlefield initiates a non-combat Showdown. Because there is no opposing unit to create a Combat, your unit does not gain the "Attacker" designation.

Since your unit is not designated as an Attacker in a non-combat Showdown, the Assault passive ability does not activate, and the might bonus is not applied.