Defeating the Balore
While fighting the Balore, when his hands go away and he starts the lasers, equip the Medusa Head Soul. This will allow you to freeze in mid-air, directly next to his eye. The fire from the laser will miss you. This way, you can smash away at his eye very easily until he is defeated.
Defeating Iron Golem
Use the Killer Mantle Soul. The Iron Golem will go down very easily.
After defeating the Iron Golem, walk to the right wall and attack it with your sword to break it down. This reveals a passage to another room. In this next room you will find the Hippogriff ability.
When you are in the Chaos Realm, equip your Roginus' Spear and the soul Erinys. When you encounter a Shadow Knight, kill him. You will gain roughly 3,300 experience points per kill. If you are near a door when you kill him, enter and exit to encounter the Shadow Knight again to keep getting experience.
When you get to the Chaotic Realm, go to the room where there is a Demon Lord and two Erinys. Keep killing them to get about 4200 experience points in this area. Also, there are coffins here. Hit the one on the upper right to always get 500 gold. Keep entering and exiting the room to repeat the process.
When you have the Fire Demon, Giant Bat, and Succubus souls equipped, you can shoot fireballs as a bat just like Alucard from Castlevania: Symphony Of The Night.
You must first obtain the Ability Soul from the Hippogryph. It allows you to perform a high jump if you press [R] while in mid-air. It is obtained very late in the game. There are some rooms in the castle that are immense and have high, hard to reach places. If you want to reach them without using the Giant Bat Soul, either alternate by pressing [A] to jump then [R] to shoot upward, or just continuously press [R]. This ability does not require magic, so your character can keep doing it forever.
When you are standing on the ground (or an enemy), instead of jumping then pressing [L] to use the Hippogryph/Hippogriff ability, you can simply hold [Up] then press [L]. You will perform a vertical jump reminiscent of the Lancers in the Final Fantasy series of games.
Restore HP and MP
Whenever you talk to Mina, she will refill your HP and MP.
Pass the large waterfall in the lower section of the castle
Equip the three God-like spirits (Curly, Devil, Manticore) which should be in the blue spirit section, also Undine in the orange section to stand on the water. Get enough distance between you and the waterfall then transform into the God form. If you gave it enough room, you will go through the waterfall, pick up the Armor, and enter a small new section of the castle.
Claimh Solais sword
To the left of the sunken ship in the Forbidden area is a room with a few fish heads that appears to be a dead end. Break down the wall to the left to gain access to a secret room. Inside is the Claimh Solais, one of the best swords in the game.
Once you have acquired the Giant Bat Soul, go into the lowest right torture room. Go all the way to the right and use the Giant Bat Soul to squeeze through the crack in the brick. In the next room is a ring called the Rare Ring. It has the same effect as the expensive Soul Eater Ring.
In the arena of the bath of Lilith and Sucubus, go up above the save point (the only one in the area ), then go right to see a toy duck.
Double soul activation
Equipping the Bat and Giant Bat Souls simultaneously will activate both Souls when just one is used. In addition, if you do not have the Succubus Soul or the Flame Demon Soul, you can still do heavy damage, while looking for hidden areas and breakable walls.
Boss Rush items
Completed too slowly: Simple potion
Less than four minutes: Valmanway Sword
Four to-five minutes: Positron Rifle
Five to six minutes: Excalibur Sword
Completed quickly, but already had the items above: Simple potion