This Seeking Tech guide will explain how you can defeat Freya in Chapter 8 of Valkyrie Profile Lenneth, which is the hardest winnable fight in the game.
This walkthrough is meant for the hard mode difficulty. It should be also be applicable in normal difficulty although it will be a lot harder to trigger the C Ending in Chapter 8.
Lawfer and Great Spear Dinosaur
To deal meaningful damage to Freya, you need Lawfer. You will also need to equip Great Spear Dinosaur, which is hidden in a treasure chest above the doorway in the Palace of the Dragon dungeon of Chapter 8.
This means that you need to play the game specifically by entering Chapter 8 with a low enough Evaluation to be able to trigger the C Ending route.
In Hard Mode, I had less than 45 Evaluation Points entering Chapter 8. Since there are three new dungeons in hard mode with 15 relic treasure chests in total, I was able to drop my Evaluation to 0 to trigger the C ending.
In Normal Mode, there is only one new dungeon with 3 relic treasures inside.
Try to level up Lawfer in dungeons, and by giving all your Experience Orb to him. You should also use Holy Drops and Golden Eggs to increase his attack power as much as possible. If you are worried about wasting resources, create a different save file.
Other Party Preparations
The rest of your party should have two mages that can both cast Might Reinforce and Sap Guard.
Make sure you master and equip the Guts and Auto Item skills for every member. You should then set Auto Item to 100% Union Plumes. For accessories, equip Angel Curio to everyone. With these, you will be able to auto-revive party members.
Freya (Ending C) Strategies
At the start of the fight, have your two mages use Might Reinforce and Sap Guard. Afterwards, attack Freya and try to set up PWS. With PWS activated, use Lawfer on the third or fourth chain in order to deal a lot of damage.
Outside of PWS, you can only hit Freya for very little, if at all.
This will be a very long fight, so make sure you have a lot of Union Plumes. Also try to remember to cast Might Reinforce and Sap Guard again when they run out. Otherwise, you will be dealing minimum damage against the boss.
The C ending will play out the same whether you win or lose the fight. So you don’t gain anything by winning other than bragging rights.