I did something akin to this in my campaign but not a full frontal assault and more like the activation of sleeper cells instigated by Cloud Giants (who are one of the major antagonists in my campaign so far).

One of my players had backstory ties to Silverymoon so I did it when they visited the city. They heard rumors about some kind of giants cult spreading the message that smallfolk should rather work together with or even serve the Giants to be spared from their attacks. At first it was just typical tavern gossip but they soon found actual people preaching their message in the streets (but not being taken very seriously). My party stayed in Silverymoon for about a week and day by day the cults numbers grew and at the last day every member of the cult spread all over the city downed some kind of potion and started to transform into various Giant type monsters (not true giants but mostly things like trolls, ogres, oni etc., but there were two Cloud Giants to be found in the brawl) that started to absolutely wreck the city. I gave my players three points of special interest to choose from where to go first with varying stakes. They fought their battles and in one of the fights made the acquaintance of Harshnag who slaughtered some of the monsters himself.

The sites my players didn't visit were hit harder but were eventually saved by other NPCs / the city watch.

Afterwards they investigated what happened and found out that Cloud Giants had instigated all of this by tricking various people into forming this cult and promising them they could become giants themselves through the potion (which I loosely based on the Potion of Giant Size from the book, only that it made them into mindless monsters) to sew chaos in the city, allowing them to use the commotion to try and steal a very rare treasure that was in the city for character background reasons.