If I attack a battlefield with Ava Achiever, can I play Hostile Takeover for a power? Hostile Takeover is a normal spell (not action or reaction), but it does have the hidden tag.

Yes, you can play Hostile Takeover using Ava Achiever's ability when she attacks, provided you pay the required cost.

Ruling Breakdown

• Ava Achiever's Ability: When she attacks, her ability allows you to play a card with the Hidden keyword directly from your hand, ignoring its normal energy cost.
• Interaction: Hostile Takeover has the Hidden keyword, making it a valid target for Ava's ability.
• Cost: You must pay the 1 Mind cost specified in Ava's ability text. You do not pay the energy cost for Hostile Takeover, and you are not playing it "from Hidden" (the face-down zone), but rather "from hand" via Ava's ability.

Sequence of Events (Showdown Flow)

1. Attack Trigger: You attack with Ava Achiever. Her "When I attack" trigger goes on the chain.
2. Activation: You resolve Ava's ability, paying 1 Mind to play Hostile Takeover from your hand.
3. Chain Placement: Hostile Takeover is added to the chain. Even though it is a normal spell, it is being played via Ava's ability, which uses the chain.
4. Resolution: Hostile Takeover resolves. You gain control of an enemy unit at that battlefield and ready it.
5. Designations: According to RiftJudge FAQ #8443, the unit you take control of will gain the Attacker designation and join Ava in the attack.

Important Note: Because you are playing this card from your hand via an ability rather than flipping it face-up from a battlefield, it does not need to have been hidden previously, and the targeting restrictions associated with playing from the "Hidden" zone (which require the target to be at the battlefield where the card was hidden) do not apply here. You can target any enemy unit at the battlefield where Ava is attacking.

Note: I could not fully verify the specific cost requirements against the retrieved rules.