Tactics for the Swamp (Fortress).

It's better to move on to practice ;)

Yes, there is practice too, but the opponents aren't as tough as in the tournaments =)
I was just sharing my thoughts that when development progresses, and if the Necromancer has developed very strongly (I just deleted my question, but Ment managed to read it), then I see some options for all castles during a siege and in general gameplay, but I feel somewhat unsure about the Swamp and Stronghold (this isn't relevant to this topic). I just don't understand how to counter a large number of skeletons if you haven't encountered them before and he has managed to gather a very large number of them. On the SoD map, it's just random, and XL sometimes throws you into a situation where you have to run for 10 days to reach his main castle, and you don't even know where it is.
Well, in general, it's not vital =) Everything is fine.

Added 3 minutes ago

Without acceleration, it's unlikely to work, and the chances are slim. But if:
1) Place Hydras closer to the gates.
2) As soon as the Hydras' turn comes, cast acceleration on them (not earlier, of course, otherwise they will slow down).
3) Skip the Hydras' turn (the speed of Chaos Hydras is 7 by default, and skeletons have 5, even if the latter skip a turn, the former will move after them => skeletons will not be able to attack in the same round).
4) Bring the accelerated Hydras out from behind the gates and attack the skeletons.
5) Next round. On the turn of the Serpent Riders (ideally the first, otherwise after the Phantom Dragons), teleport the Hydras back behind the wall.
That's how I see it, but the Hydras may suffer losses from the enemy undead, the skeletons may block them, and there are many other drawbacks. But you asked for a fantasy, so here it is.

Yes, that's a good breakdown, it can be done that way =)
PS There were also 2 other questions about heroes... maybe someone will answer. About Tiazik and Crag Hack, etc.