Some things you can add to a few of the strategies for enemies:
Giant Spider and Spider Matriarch: Berserkers make quick work of spiders due to their low HP and armor ratings, even more so if you upgrade the berserkers further. Also the Giant Spider can be OHKO'd by Wrath of the Forest if taken to the second upgrade
Skeletons: Rangers Tower Wrath of the Forest and take these out in one hit when fully upgraded.
Necromancer: When you upgrade Ranger Towers poison arrows, they will completely ignore the skeletons and focus on the Necromancer, since the skeletons can't be poisoned, This will help focus on the Necromancer so they'll die quicker
Please visit him. And my other dragons while you're at it. Ok he's actually kinda creepy, but I promise the other one is cuter!