I'm currently in the process of making ultimate ap weapons too.

I'm going for;

Triple Overdrive
Double AP
Overdrive > AP
(double ap not triple ap as triple can take 3 - 6x longer to add on each weapon)

Varuna is in the omega ruins, I usually find him somewhere around the middle of the first area.

Also if you capture one of every fiend in the Sunken Cave (where you get Yojimbo) you will unlock Don Tonberry, he will counter pretty much anything you try to do to him with an attack called Karma (I believe it hits the ammount of fiends that character has killed x100)
Set your overdrive to comrade (for all 3) and just spam steal or another skill that doesn't deal damage. once he has taken 4 steps, he will not longer counter, this is the point where you spam quick hit (he will waddle up to one character and stab him/her dealing ~10000 damage).

Naturally auto-phoenix is a must.

If your 3 characters can deal 9999 damage per hit, you will use roughly 30 - 40 phoenix downs per fight.
You can get anywhere from 200000 - 600000 ap per fight.


Hope this helps.