It's more like Zyra's ult. And the damage is probably about half Nunu's, given the fact it's not a charge up.
And no. It's not "Unstoppable" (If she's disguised it isn't visible though, until you get slammed.... so you better be looking to see whose there two of in the fight)
Keep in mind this is a champ with no Mobility skills..... other than a possible proccable on hit move speed passive.
She's going to get bombed as a mage..... kind of like Lux does. Except unlike Lux..... her big damage is local, not semi global, doesn't reduce cooldowns.