A “Fujin” is a nickname for an active skill that has a voids damage absorption effect and is used to specifically counter 
damage absorption shield.
Enemies with damage absorption shields absorb all damage above a certain threshold as health equal to the damage that would have been dealt. Dealing with damage absorption shields either requires controlling damage output to be below the threshold (which can be difficult and nigh-on impossible) or a “Fujin”. The nickname stems from the name of the monster that first possessed the effect.