First off, embark with wood, two weapons, and two full suits of copper armor, and two dwarves with appropriate fighting skills, set up your 2 man squad and await the undead.
Channel out a small area (
not open to the outside) then build a door at the entrance, set the door to passable, but NOT pet-passable, then place a garbage dump over the door.
Hit (o) and then set collect refuse from outside to yes.
Once your squad defeats 12+ undead raven corpses, claim the remains, and mark them to be dumped.
It may take a while to get all the undead in the room, but as long as your military was able to bring them down uninjured beforehand, you can kill them repeatedly and re-dump them.
Once all the remains are in a single room, forbid the door, and wait for them to reanimate.
At this point some new undead will probably spawn, just make sure they can't path to your dwarves (a passable hatch-cover set to tightly-closed works), undead will path off site eventually like any other wild creature.
Once the surface undead roam off site, the reanimated critters inside will be sufficient to hit the on site wild creature cap, preventing new undead from entering the site, said critters will be unable to leave the room and will no longer be a threat or cancel those precious harvest plants jobs.
Congratulations, you can now cut down all the surface trees free of danger, or gather plants, if you're into that (or want sliver barb for that high value dye).
*disclaimer: Won't work if RNG hates you and sends you an undead ogre corpse or something in the spring, or you selected a "terrifying" region thereby giving you giant variations of normal critters (i.e. giant ravens).
**note: Any creature you embark with (including the two random wagon pullers) that dies, will reanimate and
never path off site, they will also try to path through passable but tightly-closed doors/hatches, resulting in an undead yak cow corpse standing on top of your only exit to the outside. Murder it and dump it in the undead zoo.
In what could be its own thread, I need to ask, were clouds changed? All my evil embarks are giving me normal rain, and clouds aren't arriving (weather is ON), custom world gens.