Endgame Rule Conflict Question

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During a Munchkin game yesterday we had an endgame-related argument due to an unclear rules conflict:

I was at Level 9 and held a "Go Up One Level" card in my hand (which I can't use to get to Level 10 and win the game). After kicking down the door I got a Level 2 monster which I could obviously beat easily. After revealing the monster, another player issued a "Lose 1 Level" curse on me (effectively moving me back to Level 8). However, I chose to play my "Go Up One Level" card to negate the curse and remain at Level 9, allowing me to fight the monster to get to Level 10 and win.

Opponents said I could not play my "Go Up One Level" card then, because the rules specifically say that only cards which state "may be used one time only" and curses are legal for play when fighting a monster. However, I argued that the Special Cards section of the rules indicated I could use my "Go Up One Level" at any time.

Doesn't any time include timing for which I'm fighting a monster to negate a curse that dropped me below Level 9?