So I thought that for all enemies that you could not get around to the back of (Demon Wall, Leviathan, Tentacles from Hullbreaker Isle final boss) that all positioning requirements were not considered. I have been able to use heavy thrust (flank) and impulse drive (rear) from the front of these enemies and still got all of the damage and secondary effects from these skills. It doesnt look like you should, but I have never had a problem with this in the past.