Is their defensive set up really that bad? I mean, yeah it could be better. Narrower path and position more archer on the cliff on both sides or something, but overall it isn't detrimental to the battle plan, right? The acid pool and chokepoints protected with a few elite force, backed with a less experienced but fresh troops, while the ranges attack freely and used up their ammunition. It's just the goblin came a lot more than expected. Instead of a few stragglers running for their lives, it's the main force coming down on them.