Interfering with combat question

[EvL_Gamer]

Greetings,

I am a bit confused about the rules surrounding interfering with another players combat.

Lets say a player draws a door card and its a level 10 monster, and the player has a current strength of 12, he declares that he does not need help and can defeat the monster on his own. I want him to lose that combat, so a second later I play a card that raises the monster's strength by 5, making it 15 vs 12.

If the player then produces a one-shot item from his hand that raises his own strength by 5, making the tally now Player(17) vs Monster(15). Can I interfere with the combat a second time to try and cause the player to lose? Or, must anyone and everyone that wishes to interfere in a combat do so that first time and that time only?

To put it another way, if the player manages to survive the combat once the monster has been strengthened by the other players in the game, is he or she in the clear? Or is it that each and every time a player declares victory over a monster does he or she have to spend a tense 2.6 seconds waiting to see if it truly is a victory?

Thanks!