Your Opponent(s) go through the same 5 steps as outlined in the Attack / Player's Turn portion of a round.  The CPU opponent's ACT phase (your DEF phase) is pictured to the left.  You may use your current DEF points to play Action Cards which will defend against these attacks (assuming you have any in your hand).  The top example shows a defense Action Card being selected which will lower the CPU's attack score by 2 points (at a cost of 1 DEF point), while the CPU's damage attack score is also 2.  The actual HP your Item Card(s) will lose is the CPU's attack score minus the effect of any defense Action Card(s) you may have played.  In other words, the actual outcome of this example is HP -0 (attack successfully defended).  The bottom example shows Player 1's weapon has 0 HP, yet is still being used.  This is a result of an Assist Card being played which delays the discard of Item Cards that are reduced to 0 HP.