How to build merril in dragon age 2?

My strategy is to use fenris, isabella, and merril on nightmare. I'm doing well but merril is the weak one in the party. She uses blood magic very well but my strategy is to get enemies to not be able to attack as long as possible while fenris, archer hawke, and isabella go crazy. I will never use anders. Please help or suggestions. Sry for spelling errors. Thx.