How would you rule this attack?

[Nareau]

I'd go for 12d6, Ref save for half (DC = DC of the shatter spell). Requires a readied action to set off.

If the golem failed its Ref save, I'd have it be Prone and Entangled. Normally, I'd require a DC 20 Str check to lift the gate. I'd also allow a DC 15 Str check to tear off whatever body parts were caught under the portcullis. In the case of an unintelligent monster (golem, zombie, etc), I'd go with that option (doing another 4d6 damage that bypasses any DR).

I just like the mental image: "The golem is trapped under the massive protcullis, pinned to the floor under its massive weight. It looks up at you, lets out a soft groan, and braces itself against the bars of the gate. Then, with a sickening wet tearing sound, it pushes against the portcullis, tearing away from the lower half of its body. The torso grabs at the stones of the hallway, and it begins rapidly scrabbling towards you along the floor, leaving a trail of ichor in its wake..."

Spider

 

[ThirdWizard]

Damage from falling objects per the DMG RAW is 1d6/200 lbs mass, plus 1d6 per 10' fallen after the first 10'. Presumably friction greatly reduces falling portcullis damage or else the regular portcullis trap would do more than 3d6.

Weapons are different in that ease of handling comes into play. Even so a greatsword does do nearly twice a longsword damage in 3e, and the actual mass (not encumbrance) of greatsword probably is less than twice longsword; certainly IRL 2-handed swords used in actual combat aren't that heavy.

I'm thinking of the portcullus as more of a weaponlike effect than a falling object. I would also make it piercing damage instead of bludgeoning (crushing) damage. That's one reason I made it an attack roll instead of a saving throw. I'm thinking along the lines that it's an improvised weapon being used by the wizard to deal damage to the enemy, thus attack roll with weapon-like damage.

8d6 is not any kind of perfect number; its just a magic number. I could go up to 12d6, but I probably would have just picked 8d6 if it were an on the spot thing without others in this thread to reference. Any more than that, though, is probably too much for my tastes.