The general rule is that you may not reach Level 10 except by killing a monster. If you are at Level 9 and something else happens that would normally let you go up a level . . . you don't. You stay at Level 9. If you have a card or an ability that lets you mess with another player and get a level in return, you cannot play that card or use that ability.
BUT . . . certain cards specifically overrule this. If the card explicitly says that it can end the game, or that the level it grants can be the winning level, then obviously it can be. The corollary is that if the card does not say it can be the winning level, then it cannot be.
Note also that ANY level gained as a result of killing a monster counts as the winning level. (If a card says a monster is automatically defeated, this counts as a kill unless the card says otherwise.) If you have a character ability that lets you get an extra level after winning certain combats (e.g., the level an Elf gains for helping in combat), that level can be the winning level. A Go Up a Level card played after combat, however, cannot be, because the card does not itself kill the monster.
You cannot use a card or ability to force someone to help you if the combat is for the win. If you have forced someone to help and then the combat becomes one for the win, your helper is kicked out of the fight without penalty and you must fight alone (or ask someone to help – good luck with that). On the other hand, if you force someone to help and they would win the game, they get to stay in . . . be careful whose help you accept!