Here's how to kill them:
First, what Lunch said. Use a summon.
Second, Area damage.
Third, change areas, usually the graphics will fix itself if it
the mob follows you. If you are unlucky, you'll need to do it
multiple times.
Fourth, target a nearby creature, when it has been killed
the autotarget will target the next creature which will be the
"invisible" one if it's the only one left. Else it will be
random but it will get targeted eventually.
Fifth, sometimes a "selfdefense" AI kicks in and makes your
character target the attacker. Bad way, since it rarely
functions when you want to.
Sixth, run around and get an attack of opportunity on the
"invisible" creature. Just as bad as the suggestion above.
