any tips to beat istaroth? atm my party level above 24 its really hard to beat him
First Level up a little 3 levels even should help enough
I'm assuming you've got powerful equipment now.
So this is how I beat him (or any late game boss)
Have MC buff Irito with Super-strength then use his Hellish Cadence as his main attack have Resurrection, and Dark Heal equipped for when needed.
For Irito make sure to have an Ether Blade equipped for Irito and use Slayer Stance then Swan Slash.
For Serafina Just keep doing Healing Radiance and Resurrection with healing Prayer as back up and Mana Rain equipped just in case of mana loss.
For Jueli use Accel Boost to start then switch between Arm Slice to keep attack debuff going and Drain Slash to help keep her alive.
For Niva use Fiery Dragon (needed to maximize damage with Irito most of all) and Freeze Soul for both dmg and Atk+Def debuffs. Use enhance attack if you'd like to on top of that but I didn't bother.
Lastly make sure to put on equipment that nullifys status ailments and I'm pretty sure the main one he hit me with was hallucination most of all. Oh and don't hold back on using Awakening Pronant he's last boss so go crazy