Can Atakhan kill a Reflection to reduce cost?

Yes, you can kill a Reflection token as the additional cost for playing Atakhan, and you will receive a cost reduction, but that reduction will be zero.

The Ruling

Atakhan’s ability to kill a friendly unit as an additional cost is a mechanism that provides a cost reduction based on the sacrificed unit's attributes. When you play Atakhan, you have the option to kill a friendly unit. If you choose to kill a Reflection token:

1. Cost Reduction Calculation: The reduction is based on the Energy and Power costs of the sacrificed unit. As a token, a Reflection has no base Energy or Power cost (they are 0). Therefore, the cost reduction applied to Atakhan is 0.
2. Valid Cost Payment: Even though the reduction is 0, killing the token is still a valid payment of the "additional cost" required to trigger the "If you do" clause of the ability.

Sequence of Events

1. Initiation: You begin playing Atakhan. It is placed on the chain.
2. Cost Selection: You choose to pay the optional additional cost by selecting a friendly Reflection token to kill.
3. Cost Calculation: The game checks the Energy and Power costs of the Reflection token. Being a token, these are 0. Atakhan’s cost is reduced by 0.
4. Payment: You finalize the play of Atakhan, paying its modified Energy/Power cost and sacrificing the Reflection token as the additional cost.
5. Resolution: Atakhan enters the battlefield.

In summary, while you are permitted to sacrifice a Reflection token to satisfy Atakhan's additional cost requirement, you gain no energy or power cost reduction because tokens do not possess those costs.

Note: I could not fully verify this ruling against the retrieved rules.