Change skill_db.txt:
254,5,6,4,6,0x48,0,10,1,no,33,0x100,0,magic,0, CR_GRANDCROSS,Grand Cross
to
254,5,6,4,6,0x48,0,10,1,no,33,0,0,magic,0, CR_GRANDCROSS,Grand Cross
This changes the
// 12 inf2 (skill information 2):// 0x0100- skill that damages/targets yourself
so the selfhit is disabled.
They will still lose some HP due to the Grandcross, but it will be impossible to kill theirself.