Arizona Killer

 is a quest in Fallout: New Vegas.

Quick walkthrough
There are many paths available for the assassination. The direct approach of assaulting or sniping the president easily completes the assignment, but makes escape difficult. Speaking to the other assassin (explosives skill 50 required) can provide a bomb to plant on the Vertibird during the speech. The console outside, east of the visitor center door, can be used to either rig the AA cannon to explode (Repair skill 50 required), or shoot the aircraft down as soon as it flies in and is the fastest possible assassination method (Science skill 50 required).

Another Quick Way:

Next to the stage you will meet a soldier without his helmet. His helmet is located on a table next to the stairs below the visitor centre. Carefully approach helmet while guard is away. You will get the option (provided you have some C4 + detonator) to rig the helmet to explode. (maybe explosive skills are needed? What level?)

Rig helmet and tell the soldier where it is. He will get his helmet and return to the stage. Find safe location while president is speaking and set off C4. BOOM!! no more president.

Detailed walkthrough
(It's best to either have your companions wait or be at the Lucky 38 because as soon as you're in view, the NCR starts shooting at you.) Go to Cato at his camp. After you wait, you will have the choice of either rigging the Vertibird, (50 Explosive skill needed), or taking out the President yourself. After which Cato will give you a Trooper Costume and explain how to use it. When you're going to the Dam, be sure none of the Rangers see you, otherwise they will see right through your disguise and start shooting at you. After about a 5-10 minute wait, the President will arrive and you can either take out his Vertibird, (50 Science needed), or wait until he lands to take him out. After you kill the President, all the NCRs will come after you. Either kill them all or run. After you kill them or lose them go back to Lucius. An important thing to note is to unequip the disguise before returning.