Is there anything in the rules preventing a wielder of a flexible line weapon from coming down from above, or at an angle, so as to strike the only enemy on the field while avoiding all of allies who happen to be surrounding him?
Nothing at all.
If it's not a steep enough angle you might hit one of the surrounding allies of course.
If it's not a steep enough angle you might hit one of the surrounding allies of course.
If it's not a steep enough angle you might hit two adjacent enemies rather than the surrounding allies, of course. ;D