Order of Operations on Actions
As I've mentioned in the comments here, you add up the modifiers acting on the minion.


  The power of a minion not in play is only the number printed on it, but once in play its power includes all modifications. Its power may never go below zero.


So you add up all the modifications, and if it's below 0 at the end, it's 0 instead. If a new modification comes in, you reevalute.