Munchkin

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The rules say no card or power will let you avoid this combat, so becoming Level 9 during the combat should not affect the results beyond lessening the victim's combat strength.
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The official rule, after consultation with the designers, is that once you're in the Boss fight, you're in it, and if you beat the Boss, you win the game, period. Everything else would just prolong the game.

When a Level 10 player is in the Entrance on his turn, a Monster is drawn to become the Boss.

No card or power will let you bypass this combat! The Boss ignores any rules, cards, or powers that would take it out of the Entrance. It can never be controlled and will never ignore a Level 10 munchkin for any reason, even if its card says it will.