The correct name of this mob is Decimator Mabel Ja...the line above says "Jabel". I'd send in an official correction but my browser gets mad.
At any rate, this is one of the NMs that appear during Besieged. I haven't actually witnessed her(?) two-hour, so I'm not 100% what job it is. If I had to guess, I'd say she must be a BLM because of the massive AoE damage she inflicts, but it's hard to say because I always have my filters on during the attack. I've seen her cast -ga spells ( not sure which, saw her casting and then saw a bunch of ??? was defeated by... ) and she also has an AoE weapon skill that does about the same amount of damage.
As Besieged NMs go, she isn't too bad. She doesn't charm you or anything like that, and she doesn't seem to resist damage more than the regular mobs. Of course, like any NM, she has a lot more HP than a regular mob. The best advice I can give is probably to stay away from her if you're a mage, because it will only take 1-2 spells from her to drop you if you have less than 1000HP.