Don't take me too seriously, seeing as I don't have the book with me... But about tactics: I think that the situation that was presented came up a little too quickly for anybody to really react. There wouldn't have been any time for Elend to think too hard about the battle. In addition, I think he was bluffing a bit when he said that they were fighting for more time. I think that the main motivation he had was to burn as much atium as possible - unless my memory is faulty and he had no idea what the atium really meant. Then he was being a little foolhardy and reckless, spreading out his forces like that. Maybe he just wanted to kill some koloss?
Charononus has got it right, I think, about the arrows - that arrows don't do much to koloss in the first place, plus I don't believe that the group had enough arrows and bows to stave off the koloss for any significant amount of time, despite there being a small amount of armed soldiers.
Fatigue was probably a huge part, too. We suppose that they were fighting all night, right? The original number of atium mistings that they had in the first place was most likely small... what were those numbers again? A sixteenth of a sixteenth of an army of 10,000, even though Elend's was probably something different, is 39. It seems that atium might burn faster than we thought, or atium might give a slight stamina boost, allowing the soldiers to fight for longer, though definitely not as long as somebody burning pewter.
Their front was huge, but as to the atium logistics, wouldn't the soldiers be able to feel when their supply got low? They could conserve the last thirty seconds to run back out of the way and get back to the fighting. But that does seem like a huge failing of an army of atium mistings.
I don't remember where I was going with this. Maybe the next question or WoB to find should be about how big the Trust was and how much it had been filled, how large the atium spheres are, and exactly how fast atium burns. Then, perhaps, we could get more accurate numbers and something will surprise us all.