This guide will explain how to defeat all bosses using Rebecca for the achievement.
How to Not Miss That’s Officer Chambers to You Achievement
To get this achievements, you will have to defeat every bosses possible using Rebecca. Some bosses cannot be defeated as her, and these doesn’t count so don’t worry about doing anything wrong.
I’m not sure if Billy (or Wesker, if you’re playing Wesker Mode) damaging the boss counts against the achievement, but it’s better to be safe by either leaving him behind, or unequipping him and setting him to Idle.
There are light spoilers ahead, but nothing that will ruin your experience, I hope.
This one is easily missed, and I did on my first playthrough. Luckily it’s not that far from the start, even if you never saved.
After you can use Billy, your next task will be to restore the power, climbing to the roof through the window. Make Rebecca stay in the train to save time, and make Billy climb up. He will fall to a locked room. Keep continuing Rebecca’s story, and you’ll soon meet the scorpion-like boss.
Infected Bat (Church)
After finding 3 tablet stones and lowering the Observatory, you will exit to this new area. Far in front of you is a church, but the door is locked.
Make Billy stand on the pressure plate in the small building to the side, and make Rebecca enter the church to face the boss on her own.
Proto-Tyrant, 2nd Fight (Treatment Plant)
In the treatment plant, you will go into this room with an elevator. Billy can stay behind, so leave him to be safe.
Go down the elevator, and prepare to face the Proto-Tyrant again, the 1st fight is already Rebecca-only. Pack some ammo.
Leech Queen, 1st fight (Treatment Plant)
This happens near the end of the Treatment Plant level. To be safe, unequip Billy after the cutscene, or you can do it in the room before. The room before the boss is a long hallway with cages on the sides.
In the room, just run around and shoot the boss. Be careful, too much hits and Billy can die.
Basically, you only need to pay attention for these bosses only. A boss is Billy only, another is Rebecca only, and the final boss doesn’t really count. Personally, I recommend doing this in a normal run, because in Wesker Mode you might wanna try out his abilities against the bosses.