Arizona Killer

 is a quest in Fallout: New Vegas.

Quick walkthrough
Assassinate the President.

Detailed walkthrough
There are many paths available for the assassination.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. You now have the choice to: Tell the soldier where it is and he will retrieve his helmet and return to the stage. Find a safe location while the president is speaking and set off the C4.
 * Use the direct approach of assaulting or sniping the president once he lands, easily completes the assignment, but makes escape difficult. After you kill the President, all the NCRs will come after you. Either kill them all or run.
 * Speak to the other assassin (explosives skill 50 required) can provide a bomb to plant on the Vertibird during the speech.
 * Find the console outside, east of the visitor center door, and use it to either rig the AA cannon to explode (Repair skill 50 required), or shoot his Vertibird down as soon as it flies in, which is said to be the fastest way(Science skill 50 required).
 * Hack the terminal inside the visitors centre to alter the Vertibirds flight plan. This will cause it to go into a tail spin and crash after the president gets back on-board. However doing this will turn all the guards hostile. It's advised you hack the terminal, inform Cato you are ready and then hide nearby - away from armed guards. Don't leave the area as this will result in a quest failure.
 * Find the soldier next to the stage without his helmet and rig it to blow. His helmet is located on a table next to the stairs below the visitor centre. Carefully approach the helmet while the guard is away and you will get the option (provided you have some C4 + detonator) to rig the helmet to explode(certain level of explosive skills may be needed).
 * Run past the stage on the far left side and climb the first tower; once there kill the ranger on top quietly. You can then wait there for the president to arrive and begin his speech, then take him out long range. A sniper rifle works well for this, but Euclid's C Finder is extremely effective, and will take out almost everyone one and around the stage in one shot and will yield a very high amount of experience.

After you kill the president and kill/lose the NCR soldiers, if need be, go back to Lucius. An important thing to note is to unequip the disguise before returning!