Alright. begining of disk 4. go to gongora's mansion in Urah, go to the left door where the Royal Seal is. Break it, go down stairs and there is only one kind of monster down there. Mechanical monsters.
Just equip anti-dizzy and anti-petrify and kick their ***.
All characters will LVL UP each battle until Sed learns Double EXP, then the will level up each turn until they hit 49.
And at 49 you will learn the highest lvl of White Black and Spirit magic so you can make ming and sarah learn each in 4 battles. With Sed's Double SP you get 10 SP per battle.
This will make disk four, or at least the part i've beaten of disk 4 very easy.
YW