Tactics for Rampart.

On large maps, if I play against the AI, I only choose 1 vs. 4 (200% difficulty) and higher.

Higher than 200%? You're a madman! :D

1. They won't have those spells at all, unless it's blindness and slow on one creature – the Swamp will almost wipe them out at this stage.

What makes you say that? :) They will definitely have either expert slow or haste, which, however, is almost the same. Even a simple haste (without the Air magic school) will be enough for the Serpent and Wyvern to attack you. And all the other creatures can be healed during the battle – as mentioned above, the Swamp often has Water magic.

I don't want to say that the Swamp is stronger than the Rampart, I just disagree with your statement that it is weaker. :)

How will they beat me? They focus on defense – their damage will be laughable, and I repeat, at this stage, the Swamp will already be fighting for the Castle, otherwise it will disappear from the face of the earth in 2 days.

That's right, they focus on defense, and you'll have a hard time defeating them. Your army will melt faster than theirs, and the Elves won't shoot. That's how they will beat you.