Question: If I'm playing an online game such as World of Warcraft, how would I say the idiomatic expression "level up" in French? I know I see "monter niveau" and such, but how would it be said colloquially? For instance, how would I say,"I just leveled up to level 100."?

Answer: 
I will say: "Je viens de passer au niveau 100"! "I just reached level 100"

For a game we will more use the verb "passer" in the sense of "to go, to pass, to reach" than "monter" which means "going up"!
Adding to what Loky said (**passer** niveau 100)... To be fair, most MMORPG player don't use the purest french to express themself when they speak about their game, and some of them are young. Also as you'd expect the french academy does not bother with video game.

> 
> J'ai up [level/niveau] 100
> 
> 
> J'ai ding [level/niveau] 100
> 
> 
> 

Note 100 is not "one hundred", it's "cent" in french. "Ding" is the onomatopoeia for leveling up in World of Warcraft used as a verb. You would sound like a childish young player, don't use it yourself.

**Monter [en] niveau** is totally acceptable and this is what should be used imho.