I think a coloured cursor like in the Serious Sam series would be the most fitting solution for B<:
Full health
~50%
Almost dead
This would give an indicator for the enemies health that is suited for fast paced fun combat without adding any additional GUIs into the game like standard MMO health bars usually do.
(And it´s probably also relatively easy to add)
Sorry for posting in this old thread but i figured that it would be better than making a separate suggestion thread for it.