Indeed, the golem guards the sneaky underwater passage, not the main gate. More conventional fighters cover the latter.
The lizardfolk want the golem dealt with because that passage is the only way they can be relatively sure of reaching their kin before the hostages are used against them, but they have very little magic and no magic weapons, so can't really scratch the thing.
Not only would none of the living defenders enjoy camping in a damp tunnel all the time, pretty much all the golem has to do down there is stand in the way and punch anything going by, a task well suited to its capabilities.
Well, in this case I think we need a very strong force indeed to punch back at the golem. [By The Way], does Dissonant Whispers work on the golem? If memory serves me correctly, they don't take psychic damage, but does the other effect, the one to make them retreat work? Moving it 30' from the entrance would be huge. It would give us room to get in and surround the thing.
This is where player knowledge and character knowledge clash. I know that most stone golems are hard to hurt and carry lots of HP. Cratol as a player is listening to the lizard folk and does not think his weapon would do much against stone.
With Cratols speed if we go that route maybe he can get the golem to pursue him to give the rest enough time to come at it from behind. If it does not see you coming a few good attacks from behind it can maybe help us defeat it?