First of all, who's mount is the giant spider?! <shudder> Ick!
But to your question...Nymphs are creatures of immortal/inconceivable beauty. There is a BEVY of heinous creatures who would want to attack/slay a nymph simply for that reason alone...i.e. "Ugly" creatures who would want to put an end to the nymph's shining beauty that she radiates into the world...dissssgusting!
Your initial instincts to use trolls is a great one. I'd keep it.
Others, I would suggest/recommend (most of whom harken back, with the nymphs, to grecian myth):
- Harpies (not only would she/they want to take out the nymph, but then could turn her wood into their new roost)
- Medusae - If you go this route, I would recommend since you want "mid-level", that there is A (singular) medusa as the leader/general of this invasion.
- Hags - A convey of hags would be all too eager to get rid of a nymph..possibly/probably wanting to somehow absorb her beauty for themselves (killing her in the process, of course).
- Centaurs/Satyrs: (going back to the "classics") depending on your take on these creatures, either/or might want to abduct the nymph for their own personal pleasures. But "raise an army to kill her/take over her wood" probably not...Although...an evil centaur might make an interesting lieutenant or something. hmmm.
- Evil Fey: Depending on your use of the feywild and fey beings in your world, there are plenty of sinister fairie beings who might be more than happy to dispose of this nymph and claim her wood for themselves...maybe the wood holds some secret...some special/easy connection to the feywild...some immortal treasure (unknown to the nymph) beneath it...there's lots of way to work this.
But I'd say ONE or "a few" of the above creatures, with a team of troll lieutenants and the host/underlings being all sorts of "nasty/ugly" creatures (goblins, xvarts, kobolds, minor evil fey, etc.)
Sample Warband #1:
- Medusa - the leader (BBEG). She is after some secret she discovered about the nymph's wood. Could be the reason the nymph was placed there in the lost ages of time...You could play it that the nymph does or does not know what the treasure/secret is.
- 2 Harpies - the medusa's lieutenants, doing her bidding on the promise of gaining the woods as their own roost/hunting grounds. Commanded by each Harpy are:
- 3 Troll "captains" - each of whom commands a battalion of:
- 10 goblin archers
- 20 goblin/kobold "troops" (axes, daggers, pole arms, whatever they can get their hands on)
- The Medusa also keeps in her company a pet basilisk with another troll handler.
Total force: 98
Sample Warband #2 (motivations can stay the same or come up with something of your own. I would also say this might work better if your party is heavily inclined towards magic/spell use):
- Evil Fey prince - (BBEG)
- 3 Hags - serving as the prince's seers and advisors. Always near the fey prince with a bevy of defensive spells and a few offesnvie surprises at their command. If things are going badly (or one of their convey is slain) they WILL disappear/retreat (to the fey prince's great displeasure)...and PRESTO! instant villains to come after your party another day.
- 3 Harpies - serving as "captains" in lieu of the trolls. Each commanding a group of:
- 25 goblin/kobold troops (including archers)
- with 5 bugbear overseers/sergeants
Total force: 97
EDIT: Make it 98 and add in here an "evil" centaur champion. Either truly bad or under some enchantment of the prince or the hags' (for the prince). Releasing the centaur from his enthrallment could be an added challenge/reward for the party...and earn them a powerful ally to help them defend the wood. 
Adjust numbers as necessary.
Have fun and happy invading.
--Steel Dragons
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