How do I kill this beast? I want my 10 AP, but if I ever hit him with a spell to make him visible, he retaliates in a big way.
Vanish + Doom is your buddy.
He’s already invisible, no Vanish necessary. 8)
But someday you should try taking him down without Doom. 'Tis way more rewarding. (hint - effect magic + Atma Weapon) as long as you can survive his Meteo, you can kill him the hard way. 
If you want to get a truckload of magic on every character, doom is more rewarding than killing him the noble and violent way.
True, but it’s still something to do at least once. More rewarding in the sense of “I accomplished something” than “I got a huge amount of treasure/experience/whatever from that”
Oh, the goodies you get from Intangir. Tons of gold, and the max amount of points for gaining spells. oo bad he is not in the other world.
In the other world there are cactars and that worm thing in the desert that gives you alot of stuff.
His meteo only hits the person that kills him, so until you get enough HP to survive the attack just alternate who casts doom. If I recall, it hits for around 800-1000.
And then when you’ve killed him, you can go back to the Vedlt and have Gau leap him; when he comes back he can cast Pep Up, a Blue Magic spell for Strago! Hurrah!
Tintinabar is very helpful when you’re alternating Doom-sayers.
Although, you can have Gau use the Rhodox Rage and not see Meteo at all. Mog’s Dusk Requiem can do the same, but if he uses the spells besides Snare, it’ll kill ya.