The next Raid Boss to kill is Blood Dragon

Blood Dragon is a Beast with 19 million HP and a Counter Attack ability. You're going to need a strong front line that can survive the counter attacks and you will want a Bruxa, Scythe of Death.

Bruxa has a Aura 2 that gives a large bonus to damage against beasts.

Her evolution starts with Raven Trasnform. Raven Transform can't be farmed directly but you can get her from the current Halloween Candy event as well asfrom evolving hybrids.

For the hybrid evolve you'll need Scar, Lion of the Dark, Neo-Genetic Spiderion, and Fhagg, Orcish Deserter. 16 of each if you want to have enough Raven Transforms to evolve to Bruxa level 7.