How do I defeat Ghetsis on Pokemon Black?

I would just take items to help out my team. I never train teams evenly as i use all legendaries post game so it isnt important to me. But i managed to beat his whole team with Reshiram lv 51 and Samurott lv 72. Maybe teach maractus sunny day (if it can learn it) for solar beam/ sunny day combo. but you should be able to beat him with that team. Easily with revives and Full Restores