Castlevania: Dawn of Sorrow/Tips

Soul-Specific tips

 * Succubus: The Succubus soul deals more damage, and therefore restores more health, with higher INT stats. It also steals hit points from inanimate objects! For instance you can "bite" breakable lamps, pots, statues, and chairs. So for maximum health recovery equip INT-boosting items and souls, and go through rooms full of breakable items.

Easy leveling up
These are some good suggestion on easy leveling up:

Wizardy Lab: Merman room

 * Get the Persephone vacuum soul, and level up until you are able to use it to kill a Merman (45 hp) in one hit. Next, go to the basement of the Wizardry Lab, to the room below the room with the Ghosts and invisible walls. There you will find 16 Mermen in a line; simply go in and out of the room, using Persephone's vacuum to continually kill the Mermen and gain an exorbitant amount of experience points.
 * Another good strategy is to equip the Black Panther and Mothman souls, the Shaman Ring, and Nunchakus (acquired from Boss Rush Mode) and simply zip back and forth--each pass should take about 4 seconds. In this case, the Nunchakus are better than Valmanway because they can be used continuously. With the Shaman Ring and Mothman, you will gain 1488 exp per run; without, only 960 exp.
 * Alternatively, still with Black Panther soul equipped, use Imp soul, and wear items that enchances your INT. Activate Imp, which will suddenly set Merman's HP into just 12, then just zip through them. Oh, and of course, Shaman Ring is ever so recommended.

Iron Golem

 * In the room left of the Ghost room in the basement of the Wizardry Lab. There, use the Bat Company soul to fly over the Iron Golem, then use the Buer soul (level 9), Dead Pirate soul and Valmanway (Nunchakus and Kaiser Knuckle also work well, except that their range is short, thus risky) to quickly destroy the Iron Golem and gain 2000 exp.
 * If you don't like the large room in the Wizardry lab, you can also try the small one on the top-left-most room top-most tower in the Demon Guest House. Use bat company to fly over him and using slide back against the wall while facing him. One suitable combination is the Kaiser Knuckle with the Shaman Ring and Mothman. With  this  setup you gain 3125 exp. every time you kill him. If done right you can get in, kill, and get out in 15 seconds.

Boss Rush Mode rewards
Unlock Boss Rush Mode by completing the game and getting the best ending. Beat Boss Rush Mode under the stated time to receive the corresponding reward.

The three money doors
In the castle, three gold doors contain rare items. To unlock a door, the last three digits of your gold must match exactly the three digits on the lock.