If I attack with Ava Achiever and cast Hostile Takeover with her ability, will the unit I take control of be attacking with Ava?

Ruling: Yes, the unit you take control of will gain the Attacker designation and be attacking with Ava.

Sequence:

• Ava Achiever attacks and her ability triggers
• You play Hostile Takeover via her ability and take control of an enemy unit
• During cleanup, the stolen unit loses its Defender designation and gains the Attacker designation to match its new controller
• The unit is now considered an Attacker for the remainder of that combat

Nuances:

• The stolen unit's Might may not be added to initial damage calculations if the Showdown has already progressed past that step
• The unit can be assigned damage by opposing defenders
• The unit benefits from "while attacking" effects like Assault
• The unit remains at the battlefield as an Attacker until the Showdown concludes