bpatek said:

Does the Takedown skill deal more damage when enemy has max hp or when they have lower hp?

My crystal ball tells me that you refer to Royal Guard's Takedawn skill. If that is the case, then the correct answer is "no". ;)

You obviously confuse the enemy max HP with their current HP. "Enemy max HP" means their total HP value, no mater how much HP they have at the current moment. This is also the value you will find when you inspect your champions total stats under that exclamation mark on equipment screen.

About your question - Royal Guard's Takedawn skill damage is affected by enemy max HP. There is no different if enemy is at full HP or almost dying when he use it. To be more precise -  his ATK is also used in calculation, although this don't relate to your question.  

There are skills whose damage is affected by enemy (or the champion that use them) current HP, but this is more or less correctly stated in skill description.