If I ambush in with a Kha'Zix mutilating horror during an attack where I have one unit and the opponent also has one unit, does Kha'Zix still get the +2?

Ruling: Kha'Zix, Mutating Horror receives the +2 Might bonus.

Reasoning

1. "Alone" Definition: According to Rule 741.1, "A unit is alone when there are no other friendly units at the same location." In the context of your opponent's perspective, "friendly units" refers to units they control. Since you have moved Kha'Zix into the battlefield, your opponent still only controls one unit. Because there are no other friendly units at that location, their unit is considered "alone."
2. Ambush Timing: Ambush allows you to play the unit as a [Reaction] into an ongoing combat (as confirmed by FAQ #8743 and Rule 822). By playing Kha'Zix via Ambush, you enter the battlefield while the combat is already in progress.
3. Trigger Condition: Kha'Zix's ability triggers upon gaining the combat designation. Because you are playing into a battlefield where the opponent has only one unit, the condition "an enemy unit is alone here" is met at the moment Kha'Zix enters and gains the "Attacker" designation.

Chain Sequence

1. [Reaction] Window: You play Kha'Zix, Mutating Horror using the [Reaction] timing granted by the Ambush keyword.
2. Finalization: Kha'Zix enters the battlefield and gains the "Attacker" designation.
3. Triggered Ability: Kha'Zix's ability is placed on the chain.
4. Resolution:
   • The game checks the condition: Is an enemy unit alone at this battlefield?
   • Since your opponent only has one unit at this battlefield, it satisfies the condition of being "alone."
   • The ability resolves, and Kha'Zix gains +2 Might and 2 XP.

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