When you complete the 'Three Rocks' quest in Shevat, you will receive an accessory called 'Power Crisis' which makes your character stronger as their HP decreases.
This is an extremely useful item. Equip it to your hardest hitter (probably Citan) and keep him on low HP (as low as you possibly can, just revive him if he dies). His Deathblows will be doing thousands of damage, making boss fights incredibly easy.
This battle may seem impossible since every time you attack him, he heals 16000 HP. However, there is a simple solution.
Most of the time he will do an attack that halves the HP of everyone. By everyone I mean him as well. He starts off with 40000 HP, so it only takes 4 of these moves for him to fall down to 2500HP. This is where you attack him with a deathblow that deals over 2500HP damage.
I suggest unleashing ID for 3 rounds and using one of those attacks.
In the battling area, select Special Mode One, set the difficulty to hard, and rounds to three. Defeat every gear that is selectable to unlock Argento in Special Mode One.
In the Aveh tournament where you will face Alice's brother Dan, Do not kick his ass! Just keep defending in the fight and he will eventually give you the Wedding Dress. It has some nice stat upgrades early on in the game and everyone can equip it.
Press L2 + R2 when on the world map.
For killing Marlute the only one who can use Hyper Deathblows on Marlute is Maria, just punch it 3 times, and then keep using the X button until you enter onto infinity mode, then use any of the 2 Square or Triangle.