Nature Structures

a hero can generally deal more damage than a group of water elementals... slow and not very useful units...

Actually, it's one of the best units you can get from the portal, and often much better than praying mantises; it's a very useful unit.

In terms of buildings, I prefer Elves (though I sometimes use Tigers), Unicorns, and Phoenixes. From the portal, I take water units, especially against Necropolis; they destroy them easily.