Valkyrie Profile: Lenneth

Here's my method:

Make sure Lenneth is equipping the strongest 3 hit bow from the Divine Item section OR, if you have it, let her equip the Elven Bow.
Have Arngrim wear your strongest attack (even if it's one hit) sword.
If you have a Crimson Spear, make Aelia wear it (Crimson Spears are gained from trasmutating a Broken Spear with the Creation Jewel equipped which is gained from transmutating a Orihalcon with the Creation Gem equipped). If not, let her equip your strongest attack Spear.
Mystina should either wear a Unicorn's Horn or an Ether Scepter (try to have lots of Ether Scepters incase they break). Also, give her the spell Mystic Cross. Make sure she also has Sap Guard and Might Reinforce.

If you have first turn, just attack normally that turn (NO PWS's!!). Then use Sap Guard.
If Bloodbane gets first turn, use Sap Guard.

After using Sap Guard, attack normally but with no PWS's.
Next turn, let Mystina use Might Reinforce, then attack normally with the others while trying to get the orbs which reduce CT. Make sure you use no PWS's.

Then, after everyone can attack and Sap Guard and Might Reinforce have been used, make everyone attack normally and then use PWS's in this order:

Aelia
Arngrim
Lenneth
Mystina

This should knock alot of HP out of Bloodbane.
Then repeat my method and cast Sap Guard / Might Reinforce when you think the effects have worn off.

[P.S. Remember to never use PWS's unless you can chain everybody]

There's no point in healing in this fight so don't use First Aid, instead set the skills to GUTS and AUTO ITEM (union plume 100%). equip everyone with angel curio and power/magic bangle.

He always casts gravity blessing so just pray that at least one survives to union plume the others.

I first beat him on Hard Mode at about the same level. My party was Different, bow Lenneth, Jayle, Lawfer and Shiho but compared to yours it's near the same in terms of ending PWS damage. I also didn't use Great Magicks.

Never used Sap Guard in this fight as I didn't have it. Anyway, Might Potions, Might Reinforce was the only atk mods I used in this fight to chain my damage higher. I would advise you to try to get Sacred Javelin on Mystina and use a non-great magicks weapon. Reason for this is because Bloodbane is big enough to get hit by the full force of Sacred Javelin and all it's hits.

The goal is to at least chain each PWS on the 4th turn, 3rd if possible.

Lenneth, Mystina, Arngrim Aelia

In that order you should easily pass the 50 hit mark and Aelia's PWS is going to be -insane-.

He uses heal on every 10th turn. Hitting him with that setup by turn 8 should usually kill him, if not get him udner 30% which stops him from using Heal.