I think Sephex may have it with Lost Number.

You are what you do
I see 2 meanings here. 1: You put in lost numbers for the combination and the boss is named Lost Number. 2: The physical/magical quirk with the boss

But keep this in mind
This is either there to bridge together the riddle or could refer to remembering the safe code

Deal with these two quickly
The fight changes half-way through, and it's kinda like 2 fights, or the fact that you have to put in the code, then fight.

You're running low on time
The safe combination is timed.

Everything makes sense for that to be right.