Boulder Bash UI needs to be improved to ensure that the highlights match what actually happens when the boulder is activated. Currently some enemies shown in red, are not affected.
Do you mean that they are being shown in red, but not being hit by the oil?
The description of Boulder Bash says that the boulder has a 1.0 meter radius, which means you need to practically drop it on the enemy's head. The oil puddle radius is a lot larger than the boulder impact radius.
I reported this as well, yes it looks like a med-sized AOE spell when target/placing it, you would hit basically the target and any adjacent. But when you fire it, it is a single hitter. The new ground forming animation also is the size of what you expected when targeting, but those around the main target don't take damage even while inside the transformed terrain. It is inconsistent. I didn't read the desc, but that seems to be accurate, targeting and animation is not.