Manticoras, Minotaurs, and Dragons should be the first to engage the enemy ranks (tactics will help) and block strong archers... I usually place Medusas and Harpies a little closer to the enemy; if the enemy charges into melee against the Medusas, they might turn to stone, and if they don't (playing against a human), they will deal significant damage at range... An opponent with Medusas is quite dangerous due to this feature.
When I had 1 Harpy dwelling, 2 Troglodyte dwellings, and 1 Medusa dwelling near my castle, I didn't visit the Harpy and Troglodyte dwellings at all, so only Medusas would appear in the portal. It paid off handsomely!