Fallout: New Vegas

Before embarking on this final battle for Hoover Dam, make sure you've done everything you want to do with your current character.

This is the final mission if aligned with the NCR. After completing "You'll Know It When It Happens" and "For The Republic" you will be prompted to begin this mission. There is no turning back once you start.

If you have any companions with you that are aligned with enemy factions, they will leave your side (ie Veronica will leave you if you are fighting against the Brotherhood of Steel).

There will be a particular NCR Ranger (sometimes he has a hat, other times not) in the Visitor's Center lobby with whom you will need to speak to start the battle. You will be given a few choices as to how you would like his troops to attack, though it doesn't really make any difference during the battle.

Take lots of ammo, and some powerful long-range weapons like a sniper rifle, Gauss canon or Bush Rifle. Take lots of stimpaks (I had 14 and lots of food) and if you're playing hardcore mode, take a few doctor's bags.

Caeser's Legion enemies drop a lot of the same items:
5.56 ammo
marksman carbine
super sledge
power fist
healing powder

Along the dam there are watch towers that you can enter to take a breather, drink from a sink, or get up on top to do some sniping.

I recommend crouching (sneaking) through the entire battle while looking through your scope and picking off baddies.

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You'll get to the Legion camp (Legate's Camp) and have to battle some dogs, general legion soldiers, and finally Legate himself.

If your Barter or Speech skills are very high, you can talk him into a one-on-one battle, or convince him to surrender. Otherwise, it's you versus Legate and a dozen bad dudes all at once.
It's bonkers. Hope you still have some stimpaks.

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Once you've killed everyone, head back towards the (now closed) drawbridge/gate. Get close enough and it will burst open with a flood of NCR soldiers.

Game Over.
You win.

"Eureka!" will be the final mission in the NCR storyline. Upon completion of this mission, the game will end. This is where any support you have obtained(Boomers, Khan, etc.) will assist in the final battle. I HIGHLY recommend having two companions before getting to this point(Boone and ED-E where my companions, and they kick ass).