There are 3 dead eye levels.
Level 1.) allows the player to slow time and take single shots before returning to regular mode.
Level 2.) Allows the player to paint multiple the targets with an X. The X will show up automatically when the player puts the reticle on an enemy.
Level 3.) Is a mixture of lvl 1 and lvl 2, It allows the player to take single shots, or mark enemies (not automatic in lvl 3, you must hit R1/RB)
hope this helped!