As with any Pokemon, you need to only bring its health down and use appropriate Pokeballs. It will disappear if defeated, however, a common misconception is that once any legendary Pokemon in Black/White is defeated then it can't be caught again. If it has run away, fear not. If it's the first play-through of the game then you'll have to wait until you can challenge the Elite Four and catch Zekrom/Reshiram until you can battle any of the legendary Pokemon again (if they haven't already been caught).
However, should anyone accidentally defeat Virizion, Kyurem, Terrakion or Cobalion and save afterwards, then all you have to do is beat the Elite Four and Alder, the League Champion again. Your game will save and you'll be back at your house from the beginning. From here, simply travel back to Rumination Field via Pinwheel Forest and it will be back again.
Hopefully this clears up any doubts about defeating Legendary Pokemon instead of catching them.
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beat the Pokemon league
You can catch the main white legendary, Kyrugem, Virizion, Cobalion, Terrakion, Thunderous, Victini, Zorua, Zoroark, Volcarona, and Landorus.
acually there is but not Reshiram or Zecrom but you can get cobalion terrakion and virizion
you rebeat the elite 4
After you beat the elite four and the galactic biulding