As 
Blood Lust resolves, it checks 
Phantom Centaur's toughness. Since that's 3, Blood Lust gives the Centaur +4/-2, which makes it into a 9/1.
Then, as 
Lightning Bolt resolves, it attempts to deal damage to the Centaur; the Centaur's ability prevents that damage and removes a +1/+1 counter. This results in an 8/0 
Phantom Centaur, which then dies as a state-based action.
                    
                    
                        
Edited Callum Milne (Dec. 29, 2014 02:42:26 PM)