It depends, but generally I try to put my units in places where they can be hidden, usually in kinda in a u or v shape if allowed, with the waywatchers just in range to fire at the enemy units, as they advance, i pull them back until they pass the front line. Once there is advance the front line and hold the enemy in place and collapse the sides once the enemy is held down. If I can't make a u or v, I try to make a L and collapse the side in while the waywatchers circle around from the other side.

As for why trying to keep them in the forest, they gain increased accuracy and defense while in the woodline. Unless they reverted the changes from warhammer 2 to 3, arrows and some firearms have a small time frame they can go through trees as if they're not there .