Hmm. Some sort of custom stealth power seems like a good idea. Maybe a variation of Cloak of Shadows that works in forests instead of shadows? I'm not sure if all Forest People are immune to magic or if that was just... that one special example. Certainly we were shown Forest People scions aren't immune in one of the short stories.
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To have something put together for now, I went ahead and created a Swamp Ape (Forest Person) with Hulking Size, Supernatural Strength, Supernatural Toughness, Supernatural Recovery, Inhuman Speed, Cloak of the Swamp (like Cloak of Shadows but it works in swamps not the dark), Evocation (elements: earth, water, spirit) and Thaumaturgy (specialization: veils). The Catch was a +1 one that she's vulnerable to chemical attacks. That was a total -22 refresh.