Safer Sephiroth has a minimum of 80,000 HP; he gains 80,000 HP if you use Knights of the Round on Jenova∙SYNTHESIS, and additional HP for every character at level 99. Since you don't have any characters at level 99, Safer Sephiroth's HP will necessarily be either 80,000 or 160,000. Obviously, just don't use KotR against Jenova∙SYNTHESIS. When you use it against Safer, it alone should be sufficient to destroy him. If not, consider destroying his Wall, or repeatedly using Hero Drinks to increase damage.