I'm about to sell my copy, just because I've progressed pretty far into the game, but don't feel like grinding anymore to level up enough to feel like I can actually beat another level....
First off, ALWAYS ALWAYS ALWAYS use the ring that makes it harder for enemies to detect you. This is the best way to keep from being ganged up on, which coincidentally is the beest way to avoid death.
I strongly reccomend using the Cling Ring while playing in soul form, as well - having 1.5 times as much health than you would normally is essentially forgoing several level-ups each time you're in soul form.
The rest of the advice here is basically for players who want to grind to level up as efficiently as possible early in the game.
After beating Phalanx and making it to the Nexus, START WITH LEVEL 4-1. That is, the one with all of the skeletons with blue eyes. Each one is worth 294 souls if it's weilding a sword, or around 3/4 as much with a bow. That's enough experience to level up FAST early in the game.
Get to know their attack patterns. and practice dodging when they lift the sword up behind them. Get used to blocking their attacks other than that one.(This applies to the ones with swords - the ones with bows are comparatively easy - walk up and attack nonstop.)
Advice similar to the advice above can be applied to level 4-2, except it's worth far more!
At the beginning of 4-2, progress into the large room with 2 bridges across it, and run across the left bridge, past the phantom-esque enemies, until you reach the area most of the way across that's largely destroyed. Jump down to the right here, and you'll practically land on top of the grim-reaper-y guy there. A few quick slashes, if you're at a high enough level, and you'll take him out. This yields nearly 5000 souls, which as of when I was giong there was one level up each time I ventured into the level.
Also, the blue-eyed skeletons in this level are now worth 800-1200 souls, and are well worth killing once you get down the strategy for killing the grim reaper.
On an unrelated note, don't get greedy - remember each time you leave and come back to this level, there are just as many enemies worth just as many souls, and you'll have full health again.
Hopefully this helps someone!
-Collin