So i'm guessing their is no completed hardcore mods still. Try edditing the bosses yourself. I edited the bosses to super hard and its fun and frustrating at the same time fighting them. I'm currently fighting Proud Clod with level 70 teams with 8 Maxed HP Plus materia and hes kicking my ass. From Jenova Birth till now i have never encountered a boss and beaten them on the first 2 tries. Some has taken as many as 20 tries.
So what i'm saying is try making your own mod. you can even put some super normal enimies to encounter to liven up the battles.
PS: if you do plan to do this remember to adjust bosses difficulty for 6 HP pluses and Big Guard. So make sure bosses are doing around 50 to 190 percent of Damage and add soem random factors like effects , Barrier Breakers, and critical blows to give teh battle alittle kick.