This is an unmissable story achievement that takes place outside midgar
I was at level 50 for this but i am a little overlevelled due to grinding for other "disc 1" achievements.
DW has 30,000HP and cannot be hit with physical attacks until its chest is opened and it is counting down to diamond flash.
Equip and use your best summons and magic, and heal whenever necessary, big guard can help but runs out quickly. There isnt much to this battle really.