I have a Kha'Zix on one battlefield, in the other one the opponent has Ferrous Runner, another unit, and a hidden. I played Void Assault to go in with Kha'Zix and move Ferrous Runner to base. So I get the +2 or no?

To determine if Kha’Zix, Mutating Horror receives the +2 Might bonus, we must look at the battlefield state at the exact moment Kha'Zix gains the "Attacker" designation.

Ruling

Yes, Kha’Zix, Mutating Horror receives the +2 Might bonus.

Reasoning

1. "Alone" Status: According to Rule 741.1, a unit is "alone" when there are no other friendly units at the same location. For the purpose of this condition, "friendly units" refers to those controlled by the opponent.
2. Hidden Cards and Unit Status: Facedown cards are considered facedown game objects rather than units. They do not gain unit properties until they are played or flipped. Therefore, a hidden card does not count as a unit present for the "alone" condition.
3. Timing of the Check:
   • Once Void Assault finishes resolving, Kha'Zix is present at the battlefield and gains the "Attacker" designation.
   • At the exact moment Kha'Zix gains the "Attacker" designation, the game evaluates the battlefield state. Since the hidden card is not counted as a unit, if the only remaining opponent entity is that hidden card, the "alone" condition is met.
4. Condition Met: Because the condition "an enemy unit is alone here" is satisfied at the moment Kha'Zix gains the Attacker designation, the ability triggers and Kha'Zix receives the +2 Might bonus.

Chain Sequence

1. Play Void Assault: You play Void Assault, creating a chain.
2. Resolution: Void Assault resolves, moving Kha'Zix to the battlefield.
3. Attacker Designation: Kha'Zix gains the "Attacker" designation.
4. Trigger Condition Check: The game checks: Is an enemy unit alone here? Because the hidden card is not a unit, the condition is satisfied.
5. Ability Resolution: Kha'Zix's triggered ability is placed on the chain and resolves, granting him +2 Might and 2 XP.

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