The level varies depending on your own combat level. They don't drop anything relatively good, so your best bet is to run away and continue woodcutting.
Now since the random event update, you can only find the Tree spirit in the Enchanted Valley. To access it, simply woodcut any tree there.
When used to be fought, if you were woodcutting and weren't wearing capable gear for fighting, it'd be best to just turn on protect melee prayer and use your axe to kill it.
A very angry nymph.
Snapdragon seed, Rune hatchet, Rune javelin (5)
Bronze hatchet, Iron hatchet, Steel hatchet, Mithril hatchet, Adamant hatchet, Grimy herbs, Nature rune (5-22), Cactus seed, Poison ivy seed, Toadflax seed, Dwarf weed seed, Banana, Nature talisman, Knife, Avantoe seed