Personally I wonder why they don't let you target points instead of models. You can still take damage from an invisible monster, so you should be able to target them as well. Then again, many HNM are just invisible and still at the spot they died, so you'd be able to target those as well.

I guess SE should have made a variable called "targetable" when they began programming so they could have solved all these problems by looking if it is true.