Does En Garde get additional +1 might if it is attacking a battlefield that is held by someone else riftbound?

Yes, En Garde can grant the additional +1 might bonus if the targeted unit is the only unit you control at that location, regardless of whether you are attacking or defending.

According to the official ruling in the Riftbound FAQs:

• The bonus condition checks if your targeted unit is alone among your units at that location.
• Controlling units is separate from controlling locations.
• The bonus applies based on whether the targeted unit is the only unit you control at its location, not whether you control the location itself.

Therefore, if you play En Garde on a unit that is attacking a battlefield held by an opponent, and that unit is the only one you control at that specific battlefield, it will receive the full +2 might bonus (+1 base, +1 additional).