When to gain winning level in combat?
 

It's been ruled in other threads that a Munchkin does not gain levels and treasure until after combat, such that the other monsters must be killed/snuffed before the Munchkin gains the levels and/or treasure.

Consider this scenario:

Munchkin A is at level 9 and in combat with a random monster. His bonuses easily defeat the monster.
Another Munchkin does not like this so before combat ends, he plays a Wandering Monster to add an Undead monster, to which others add other Undead monsters (using the to-be-included Undead rule). All in all, the Undead horde proves too much for the Munchkin and he must Run Away.

As per the ruling from other threads, the level and treasure from the first monster is not gained because Munchkin A did kill/snuff the other monsters. Correct?

Re: When to gain winning level in combat?
 

Exactly. If you can't beat the total combat strength of a group of monsters, you need to run away from all of them, and running away means you can't get any levels nor treasure. You even have to run from the monster you could beat.

Re: When to gain winning level in combat?
 

The rulings in this forum you are referring to are only supporting the rules, which say the exact same thing: You must fight all the Monsters together, and you only collect the rewards after defeating the Monster(s). The main player in combat has to declare that he's winning the combat and give people ample time to stop him. If people decided to stop him, he didn't win. Just by facing a Monster one could beat does not mean that player has won right away.

Re: When to gain winning level in combat?
 

To clarify the last sentence: If you actually kill a monster but then have to flee the combat, you don't have to try to roll to Run Away from the monster you slew -- because it's dead -- but you don't get the rewards, either.

Re: When to gain winning level in combat?
 

I think it's also important to mention that the only way you can 'actually kill' a monster without winning the combat is via some sort of automatic kill provided by a card. Such as The Mighty Germ being killed by a Halfling, an Epic Orc munching a level 1 monster or using the Potion of Halitosis to kill the Floating Nose. The original question involved beating a creature with your bonuses alone, which seems impossible to do while still having time for other monsters to enter the combat.