@captain_crunch @dasheek You missed it because the chapter glossed over it; they are in formation. Archers and casters in the back in relaxed formation, warriors up front in loose formation.
For reference:
Tight formation is when you are basically shoulder-to-shoulder
Relaxed formation is when everybody is about one arm-length away from each other
Loose (also known as skirmish) formation is 2-5 arm-lengths away from each other. Far enough away that you can't actively cover for your buddy in close combat, close enough that it takes you less than a second to get to him.
Any further than that is no formation.
Page 28 of this chapter shows them to be on the outer edge of Loose formation - somewhere on the order of 5 arm-lengths.
Tight formation made sense back when the enemies were comparatively small and a shield wall could hope to stop them and cover for your allies - check chapter 23 and see that all the monsters were
roughly human sized. Against monsters 3 or 4 times your size who can fly (or at least jump over you without trouble) a tight formation where everyone is close to each other does nothing but pack more people into the kill area of its strikes.
Now, it's overly focused on Will's part in this so it doesn't show it, but the smart thing in a situation like this is to have three groups of front-line fighters. First one makes the initial charge and fights until the situation resolves into a stable battle, second part is right behind them and moves forwards as needed to plug holes and provide assistance, third hangs back and splits their forces between watching the flanks and recovering the wounded.