FormerlyHemlock
Hero
A quick Google search suggests 60 feet is long range for an improvised weapon. Can a book holder confirm? If so... Even with disadvantage he's still likely to hit, so I think your wizard is still boned.
Okay, great. If the Tarrasque can kill a first-level wizard*, then it must be at least as powerful as a housecat.
* Unfortunately it can't catch up to him, because if it's spending its action on throwing rocks it wasn't spending it on Dash. So it has to hope the wizard doesn't notice it casually slouching into 60' range with a giant rock in its paw.... but at least it can theoretically kill the wizard, in a white-room scenario. ;-)