To trigger the secret fight with two red (orange?) robots in the elevator you need to finish each of the three robots with a different weapon.

Guide

1. Start the game in the Standard or Causal difficulty.

2. Progress through the game until you get to the elevator.

3. As soon as the fight damage all three enemies to low health but don't kill them.

4. To trigger the secret fight simply finish each of them with a different weapon.

5. Finish one with the Pistol, one with Diana's hack ability and the other one with the green gun (trap gun).

6. Kill the new enemies in whatever you want.

7. Now progress to the end and kill the boss to get the No.2 ending.