Alright, from Billy's point of view the spiders where a purposely ineffectual terror attack. Weaklings. So, he's going to try and modify his before bringing it back. Give it some better armor, attach a few extra legs to it's back, attach fangs to all of it's legs. You know, give the enemy a little poetic justice when they show up again.
Logic goes like this: when you animate a skeleton, there is nothing holding it together because everything that was has rotted away. So you could add things to it if you wanted to, because the only thing really holding it together is the magic.
Now it's not going to be a super powerful tank spider that can take anything and win but with the modifications it will, at least, be viable in a fight.
Hey son, wanna' learn how ta' make witch balls?