Once you are in the safe room, poppy will start talking about how to use the surrounding area to defeat CatNap, you'll need to put two of the power cells into
the walls before she continues her dialogue, one in the socket that powers the energy you can take with your green hand <-( leave this one in the whole fight ), the second you'll need to put into one of the sockets that power a steam wall, once she moves on talking about the power receivers, go ahead and take out the power cell you just put in that powers the steam wall, from here on you wont be putting the power cells anywhere except the main wall in the center with the 4 sockets, now continue through poppy's chit chat about the power receivers on each side , this is the only way you are going to use the steam walls on either side to fend off the real CatNap, along with your flare hand to check for the real one, you'll be switching between the green hand and flare hand frequently throughout the fight.
Tip1: If CatNap is opening the hatch above, any CatNaps coming through the halls at that time are illusions. ( i still flared them after anyway )
Tip2: when you need to open the gate on the right to get to the power receiver, switch to your green hand, open the gate with your blue hand and don't let go, that way the gate will not close before you get to the power receiver.