Arizona Killer

If you're feeling sneaky, you can make your way up to the elevated speaker's platform on the north side of the Visitor's Center. Kimball uses the eastern-most stairs to get up there. Plant some C-4 Plastic Explosives on those stairs, wait for Kimball to arrive and get close, then detonate it with your Detonator. Easy peasy, if you can get into position without being seen.

This is probably the cleanest way to do it, and it doesn't require much from you. Sneak into the Visitor's Center – watch out for that Ranger with the dog patrolling outside! Inside, hack the Terminal. You'll need an average Science skill for this. Access the 'Hoover Dam Security Report', 'Security Duty Roster', and 'Engineering Report' to get the lowdown on security and Kimball's vertibird. The key option here is 'Flight Control Update' – select 'Modify Update Settings' to rig the vertibird. Once Kimball gives his speech and takes off, he'll crash. Boom. You'll need a Science score of 50 for this method.

If you want to get your hands dirty, you can try to stealth-kill an NCR Engineer inside the Visitor's Center. The one standing alone north of the entrance is a good target. Loot their NCR Engineer Jumpsuit – you'll need it. Put it on and head upstairs, then go out the south door to the observation deck. Find Cato Hostilius (he's usually in the crowd north of the Visitor's Center) and tell him you're ready for the President to show up. Once Kimball's vertibird is incoming, head back to the Visitor's Center. Go east around the building, past an NCR Trooper and some barricades (watch out for Ranger Grant back here!), and climb a ladder to the roof of the Visitor's Center. The Engineer Jumpsuit lets you get up here. Wait on the south-eastern part of the roof, away from any patrolling Rangers. Once the vertibird lands and the President disembarks, approach the machine and 'Plant the bomb'. This requires an Explosives skill of 50. If you got the bomb from Cato earlier, you can plant it now. Wait for the speech, and enjoy the show.

Still not enough options? Head upstairs in the Visitor's Center and exit through the southern door on the second level to reach the observation deck. Turn east, then north-east to find a Console. Interact with it and select 'Examine Gun Controls' to rig the anti-aircraft gun to shoot down Kimball's vertibird. This needs a Science skill of 50. Alternatively, pick 'Examine the gun circuit board' to rig the gun to explode during the speech, which requires a Repair skill of 50.

The classic 'shoot 'em in the head' approach. You can do this openly or sneakily, but with all the NCR Rangers around, a long-distance shot is probably best. Getting onto the landing pad (follow the steps in 'Bombing Bear Force One' to get up there undetected) gives you a great vantage point. Just be warned: the second you draw your weapon, Kimball and his guards will try to escape to their vertibird. Luckily, he has to run towards you to get there, so one well-aimed shot from a good rifle like Ratslayer should do the trick. Plus, this way, the rest of the NCR won't immediately know it was you.

No matter how you manage to take out President Kimball, there are a couple of things to remember. He won't show up until you've spoken with Cato Hostilius, so make sure you do that first. After Kimball is dead (by whatever means you chose), get out of the Hoover Dam area as fast as you can. Once you're clear, fast-travel back to The Fort. Make sure you ditch that NCR disguise if you were wearing one before you report back to Caesar.