Duel map for Heroes of Might and Magic III.

Let's go through it step by step.

Well, you know, I don't know what's easier to break through: 55 snipers or a Castle defended by at least two Angels, which is on the second week, playing on 200% difficulty and starting without any Forts.

Yes, let's consider the 200% difficulty without Forts.

Well, let's say even if you manage to do that (I doubt it),

It depends on which Castle.
Playing as Necropolis, on this map, by week 2, you have 11 Vampire Lords (which is clearly too many for a Castle), who can kill Griffons without any losses on the approaches to the Castle. The hero can easily visit the Guild and have Stone Skin, Shield, and Bloodlust. With at least one of these spells, 11 Vampire Lords can take a regular Castle on week 2 without losses, and if you manage to find Attack or Defense spells, it's even easier, and don't forget about the 6*1 skeletons, which can be used as cannon fodder. The main thing is that the Griffons don't have high morale on every turn. If there's a group of Royal Griffons, which is always known before the battle, it's a bit more difficult, but it's quite often played.

then 2 Angels won't kill 15+25+15 snipers, especially since they won't reach them in one turn without tactics and will be shot down in the first round.

But here you are definitely wrong; the Angels from positions 2, 4, and 6 can reach the snipers. 11 Lords and 2 Angels can break through the garrison without losses (again, if the snipers don't have extremely high morale, then the Angels might fall). And the hero might also have Haste, so the Lords can reach them as well.

And if the Castle is defended by three Angels? That means you've lost, because 150 Griffons on the map at the start is a lot.

Yes, in reality, it's not possible to take it on week 3, only in theory, but on week 3, everything will be fine; you can wait, capturing dwellings, mines, Gnomish Stronghold, a Tomb, and killing everyone on the map.