First, a naming convention. Like there's "Call of the Wild" (instead of Summon Wolves), I think it's nice to call this skill "Call of the Woods" instead of "Sylvan Nymph". Notice that this is one of the few pets that doesn't have the proverb "Summon" in its name.
Anyway, about something other than superficial changes:
The Nymph needs more power. I think it should get some buffing or debuffing spell at level 5, which increases per level. Not sure what though.
As for Overgrowth; it completely sucks. It absorbs like 200 damage at max, is cast at random intervals, it disappears if the damage is exceded (which is pretty much after 1/3rd of a hit on Legendary) and it is only cast on the player. My idea is to change how the Overgrowth shield deactivates; instead of it deactivating after the damage is exceded, it comes with a 20 second timer that increases by 1 second per skillpoint.
As for Nature's Wrath, the damage seems decent, but it's only used 1 in every 4 shots or so. I suggest to reduce the cooldown by 70-80% or so, else the skill will hardly be used.