At 9th level+, how would 1-3d6 con damage kill them? It's a 10' pool of green slime; swim to edge (move action) exit pool (move action as standard action), take 1d6 Con. 2nd round, take 1d6 con, then one burning hands kills all the slime; it has no HP total: "Anything that deals cold or fire damage, sunlight, or a remove disease spell destroys a patch of green slime." Most likely they'd take 2d6 Con damage. Their wood and metal equipment would likely be severely damaged, though, taking 2d6 damage. It's nasty, but not independently lethal.