Actually, that’s not 100% accurate. Yes, Frost damage is what freezes him. As of patch 5.1 it takes 20 hits with frost damage, down from the original 200, to freeze Viscidus. That, however, is only the first part of the battle. Once he freezes you have 30 seconds to hit him 30 times, down from 75 in 15 seconds, with Physical Damage, spell damage does not work here it has to be Physical Damage.
If, within that 30 seconds, you can not deal 30 hits of physical damage to him he will thaw and the process resets. The physical hits done before are reset.
This is why this boss fight is a problem. While classes like Druid have Cat Form, who’s auto attack speed alone is 1.0 (1 attack a second) before haste, with multiple moves that deal “Physical Damage”, not all classes have that benefit.
Take Frost Mage who, even with the fastest 1h weapon equipped (dagger), they have an Auto Attack speed of 1.8 (1 attack every 1.8 seconds) before haste. Thats it!
Take Prot Paladin, as I mentioned above. Before patch 8.0 and the changes made I was able to BARELY scrape a victory out of this boss, now I can’t unless I go and acquire a niche trinket such as “Terracotta Fragment” or such to garner extra auto attacks. Most of these trinkets are not even that easy to acquire, either dropping from rare elites (Actual rare elites, not the WoD-BFA era rares) or from things like Archeology. Many, such as “Barov Peasant Caller” were even removed from the game.
The point I am making is that, while some classes can breeze through this fight, there are a lot as of 8.0 that have a far harder time with this fight now given the changes to secondary stats and mechanical changes.
The followup point I made was that Razorgore was solo able by most everyone if you played your cards right before his change. The fight was tight but doable and yet it was changed for ease of access. While Viscidus may not be a required boss, and is optional, there are pieces of transmog and other items locked behind this boss and it is by far the least solo friendly fight in all of Vanilla.