This is definitely not an easy one unless you get lucky. We went 0/6 before we won. Our final winning set up was WHM/smn BRD/whm(ballad+1) RDM/DRK Nin/drk war/nin and mnk/nin and we tanked near the entrance to the tunnel.
I kept barthundra up the whole time which seemed to help resist his stunga and Thunderous Shriek moves. and the rdm and drk tried to stun all of his -aga moves.
Whenever he summoned a weapon, the two melees broke off and fought it with the bard usually there to cure them.
Another thing to keep an eye on: ERASE your tank IMMEDIATELY after Cackle goes off. Magic defense down sucks, majorly.