Ability damage is the way to go. With a 13 dex, 6 int, and 16 wis, all these stats can be the target. Int is easiest.
What you need is the following:
-rope trick, so you can prepare and set up an ambush. It's extraplanar, so defeats empathic senses. What the hell... make it a magnificent mansion.
-some wizards with access to Ray of Stupidity (lvl 2, 1d4+1 int damage, ray, close range, no save, SR: yes). 4 should be enough (1 rolled a 1 to his hitroll, others roll minimum damage) to ensure the beast is left in a coma. Ray of stupidity is not capped to 1 int like touch of idiocy or bestow curse.
-Sit in magnificent mansion until gargantua gets near.
-on each wizard, cast improved invis, mindblank and fly.
-wizards exit mansion, and cast their rays.
-warriors charge in and hit it until it dies.
optional step: reduce its HD and make some simulacra (Gargantua mounts for everyone!)