For this achievement you have to play the game up until the point where you encounter Ocelot for the first time, There is no battle needed as the fight just plays out in a cutscene. If you are trying to get the Achievement for preforming the Ocelot Time Paradox, Then pull out your knife and stab him after the cutscene rolls. He will be knocked out so he is a sitting duck
if you came here for the tips on the Ocelot boss battle later in the game,
The Boss battle consists of Ocelot trying to be a showoff, He does very stupid things and is pretty much a Gun duel between you two. The boss fight is not hard, But there is a few tricks that will make Ocelot even easier, First trick I use is shooting the Hornets Nest, This will slow him down and make it easier for you to get a few shots in. Also if you shoot off his hat, He will frantic and make it again easier to get a shot off. Be aware if you want to obtain all the Camos in the game, You must beat him non lethally which will unlock the Animal Camo.