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.