Lina and her friends are most definately higher than 12th-level.
In episode 1, Lina casts Dragon Slave. This is no mid-level spell. Gourry, when he actually fights instead of tripping over himself, kicks unholy ass. If I had to guess, these would be the stats of the characters circa the end of season 1:
Lina: sorceror 20
Gourry: fighter 17
Zelgadiss: shaman 16 (made-up class)
Amelia: cleric 12
Sylphiel: cleric 13 (when she's not being a moron)
Rezo: cleric 30
Shabranigdu: CR 40 demon
There's actually a Slayers d20 product out there. Xellos is like a CR 40 or something. It's insane. The whole party is well into epic by the end of Try.
God, I'm a nerd.
That sounds more like what they were doing in the shows then level 12 or so.