It takes 3 playthroughs doesn't it?
Takes 3.5 if you are looking to do it legit (mainly because some of the Demon's Souls you need 4 of to make and get everything). If you choose to do everything legit, like I did, here are some tips.
First, make sure you know about the gecko's and where they are, since you have a limited amount of them each playthrough (some are in easy spots and others in tough spots). You need them since most of the pure stones from them and the farther in each level you get the higher quality stones you will find from the gecko's. Farming for the lower quality ones can be a pain, but its like any other grinding in an RPG. Hardest pure stone to get is the pure bladestone (has a really low drop rate and only two black knight ninja's can drop them in the game).
Second, try your best to get white character tendency on your first playthrough (I had to get mine on second and was a pain). For this you have to kill the black phantoms in each world to raise the character tendency. This is needed to get one of the rings. After that, going black tendency is easy, but getting white is a pain.
Third, don't let the 3.5 playthrough's look hard. The game is meant to get harder for each playthrough, but once you know what to do it isn't so bad. Believe on the second playthrough the difficulty (damage you take per hit) goes up like 60% from the first playthrough. The catch it though after the second playthrough each one only goes up by like 8%, so it's much less. Once you know the patterns and such for each enemy and boss it doesn't matter too much.
When I went through I made sure to do a lot of the stone grinding, and trying to push my character to white tendency as soon as possible. This made it so near the end all I was really doing was going through just getting the demon's souls I needed for weapons and spells.
While I did enjoy the game, I think a lot of people praise it a little too much. Good luck if you do decide to get it by normal means. I never did any friendly summons when i played. I only did the evil ones to get the Monk wrappings, just to get it.
Also, off topic but there are 140 people with platinum on Lost Planet 2 now, not 7. Granted, it is still a hard platinum (mainly cuz of not resetting the times online). More people will get it along with Spelunker 100%.