Well obviously the first thing you need to do is to beat the Mohg, The Lord of Blood. You will find him Mohgwyn Palace which can be travelled to via the portal in Consecrated Snowfield. You'll need Dectus Medallions to access the region.
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Secondly practice around with some of the game's toughest foes. If main bosses are dead, explore other mini dungeons and fight the bosses to get back in shape. Explore other weapon types and get affilated with them; so that you can learn their know how as well. Who knows what kinda weapons you might find in the DLC.
Lastly make sure you have Bloodhound Step. I have a feeling it might come in hand