Tactics for playing against Sylvan.

The situation with the portal has changed dramatically since the release of ERA 3.51. Now, the AI no longer builds a portal in its castle to prevent troops from accumulating there.
Moreover, the ERA team promised to fix this AI's "reluctance" to recruit troops from the portal. If they do, who knows, maybe the offline gameplay tactics against Nature will undergo radical changes :)

If that happens, then you can surrender immediately... The portal is the most powerful thing Nature has, in my opinion... Without it, I wouldn't say it has a strong army, but with mantises... water elementals... hives...
In principle, Nature is not very good at dispelling spells (although nothing prevents you from leveling up Life), its main magic is focused on summoning and increasing the number of creatures. For some reason, I think that Order has a good chance against Nature - banishment, a couple of curses, hypnosis... And dragons are weak against titans... Death - perhaps, with a tactician and by loading up the Nature units with curses (antimagic on everyone - IMHO, not the best option), while bypassing the necromancy ward. Strength can only counter with a large amount of Strength units against a certain number of Nature units, and with barbarians and immortals :) Life will act similarly to Death - tactics plus abundant casting of spells, only this time on its own warriors. Chaos can throw black dragons closer to the dwelling and quickly eliminate them, and at the same time, the hero. Again, you can cast some nasty spell.
But in principle, as was said earlier, prevent Nature from building a portal and take down the hero as quickly as possible... And what is more convenient for others is better known to fans of other factions, because I mainly play AS Nature ;)