I found that an easy way to take on Safer Sephy (second form) was to have someone with the "Destruct" (I think) materia, a person with KotR, and a person with mime. When he puts up his Wall, get rid of it with the De-barrier (or whatever spell it is), cast KotR cast, then use any other summons as necessary. After the barrage of KotR attacks, he shouldn't take long to take down.