Bring an interrupt, both for Bear Form and Purge Signet when he tries to rid himself of hexes/conditions. He's got half skill activation time.
Standard anti-melee tactics (blind, hexes, snares) will render Magni nearly harmless.
Magni is Old Norse for "strong".
Magni is the name of the son of the thunder god Thor, in Norse mythology.
Magni's name is likely a reference to M. Bison, a boss character in the Street Fighter series of video games. The connection is further proved by what he says when one repeats the quest. He mentions that the new tournament is his "Championship Edition", which was part of the name for one of Street Fighter games: Street Fighter II: Championship Edition.
Magni's dialogue of "Show me how defenseless you really are!" is a line from Breaking Benjamin's song "So Cold."
When under the effects of the Boreal Tonic you resemble Magni.