My opponent has a 4 might unit at a Forbidding Waste. I deal 2 damage to it. Then, I move in with a unit. Does his unit die immediately, or is there a window for my opponent to react before it dies?

Conclusion: The opponent's unit dies.

To determine if the unit survives, we must evaluate the board state and the sequence of actions.

1. Analysis of the Unit's Might and Damage

• Initial State: Your opponent has a 4-Might unit at Forbidding Waste. Because the unit is alone, Forbidding Waste applies a -2 Might penalty, reducing its current Might to 2.
• Damage: You deal 2 damage to this unit.
• Result: The unit currently has 2 damage marked on it and 2 current Might. Per Rule 323.5.3b, this unit is at lethal damage.

2. Constraints on the Move Action

Per Rule 144.1.a, a Move action requires a Neutral Open state. Because the unit is currently at lethal damage, it will be destroyed during the Cleanup step following the damage, which occurs before a new action (such as a move) can be initiated.

3. Battlefield Control and Penalty

Per RiftJudge FAQ #10921, a unit is considered "alone" if there are no other friendly units at that location. Moving an enemy unit into the battlefield does not remove the -2 Might penalty from Forbidding Waste, as the opponent's unit remains the only friendly unit present.

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