Yes, that exactly is the one thing I don't like about the whole Slayers universe, like I said before. You would think a spell that can level whole towns (like a nuclear bomb) would be somewhat difficult, demanding, and thus a rare skill, but nah - it's there like a fireball in other stories; for any magic user showing a little interest.

What comes to Amelia and Zelgadis, let's not forget Zelgadis was a villain at some subleader level in the beginning, and a direct underling (and creation) of the big bad guy. It makes sense he knows some devastating magics. Amelia, however, is some spoiled kid who got a bright idea of saving the world with justice of her own making.

Well, Slayers never was any deep series and thus skills and details come and go according to what is needed by whatever flimsy plot currently happens to be in control of the characters.