(known & tested) Water Elementals on the Haunted Isle (off the NW coast of Stranglethorn Vale)
(known & tested) Frostmaul Giants in the trench on the south end of Winterspring
For Fire Immune,
(assumption) Fire Elementals in Blackrock Depths
(assumption) Fire Elementals on Fire Plume Ridge in the center of Un'goro Crater
I assume these elementals wold also be immune to Polymorph.
For evading mobs...
I find a cat a lot stuck in a large tree or large rock. He's just off the mountain ridge on the south side of Badlands a little west of the Trogg area
In Winterspring, you can drag a nearby cat up the bottom of the Iceflow hill for the Winterspring Trainer rep turnin. As the cat tries to go up the hill, he starts evading--apparently they can't climb the Iceflow.
(Edit, one more...) Jump in the water an aggro any pirahna or whatever (some in STV rivers), then get out of the water causes it to go into evade mode