Necros have no regard for treaties whatsoever, and so occasionally show up at the beginning of summer regardless of your training levels. They're easy to deal with though: Build a door ASAP and dig out a facility with underground farming and workshops. Install the door, and if the undead show up, just get everyone you can inside, lock the door, and wait it out.
Now, if you don't have an aquifer in the way, you can dig down to the caverns to release spores so grazers you managed to get indoor can survive. Caverns mean wood, although they also mean dangerous critters. Get some wood and block the cavern entrance with an airlock (two raising bridges with space in between). Once you've got some wood you can make cage traps to protect the cavern entrance, as well as cage traps to install in a tunnel you dig. Given the small number of undead, I'd start with the tunnel, install the traps, breach the surface, and wait for the undead to file in. Be prepared to cancellation spam about lack of cages, though...
You can also dig a tunnel, install a raising drawbridge in it, breach the surface, and wait for the undead to path into the "fortress", trying to close the entry as soon as the first one is through. Your newbies don't have that much of a chance against undead that are stronger, don't tire, and don't have vital organs, though, unless you can gang up on them one at a time.
You don't need an anvil to make weapons, though: "rock" swords are made from obsidian plus a wooden handle, and you can get obsidian from an obsidian farm.
Waiting for a caravan won't work, because they, unlike visitors, have the sense to keep away while you're under siege. Rabble such as monster layers might show up to have their body parts spread all over the surface, though (In my current game the undead showed up in the spring after embark and slaughtered about a dozen questers/monster slayers, with the last straggler bagging 3 or 4 before it finally was done in by a military visitor).
Also note that sieges last one season or one year, after which they rush for the edge (except, probably, campers).