Had a strange thing come up last night, and looking for input on if my ruling was correct.  

Party, fighting a Flameskull (resistance Necro) and summon a shadow (damage nercro  & Strength drain).   Shadow hits Flameskull for 4 points Necrotic damage and 2 points strength drain.  I rule Flameskull's resistance reduces 4 damage to 2.   Then the tricky part.  I decided that the ability was "Damage Resistance" with no special/additional text,  ability drain was not damage, and the Flaemeskull takes strength drain (1-2 drops to zero) and is dead.  

Was I right?  

Indeed, the ability drain is no part of the damage:

http://www.sageadvice.eu/2016/03/08/if-a-shadow-rolls-a-critical-does-it-do-2d4-strength-damage/

Actually, your players did quite a clever thing.

Thanks Filcat!!!

I admit it was late and I was tired, but I do both a google check and a search within Sage Advice and missed that tweet.