In my world, skeletons are a thing. They're raised from existing corpses under the control of their "master", they cannot talk and will usually attack whatever they see.

Obviously, there's magic involved. But I don't want magic to be part of the whole process. Let's say the magician behind the skeletons control is simply acting as a puppet master, pulling strings from dead people's skeletons.

How can these skeletons be challenging and a little realistic?

Bones are kinda resistant, but not that much. I guess the mouth would be full of various infections considering this is a corpse we're talking about.

Let's assume the puppet master can provide them an above-average speed (considering he's the one pulling the strings) so they can hit with a pretty good momentum. The puppet master are also very good war tactician, so the skeletons aren't just empty minds running to the battlefield.

These skeletons can be animal or human skeletons. But obviously, it's easier to find human corpses than grizzly corpses.