La Mulana is.... an unfortunate game. It is very interesting and has a lot of potential, but it commits many gaming sins that can't be overlooked. This is a NES era kind of game that is designed to force you to use some kind of guide when playing it (think castlevania 2). If you respond to this, don't just say 'the game is supposed to be hard'. Dark Souls is my favorite game; I know the difference between a game that is difficult because it's designed well, and a game that is difficult because it is designed badly.
-Very little feedback is given for many of the puzzles. Many times if you solve a puzzle, the game simply doesn't show you what exactly you unlocked, or where. Sometimes solving a puzzle in one area will unlock a door or something in an entirely different area.
-Although clues are given, many aren't helpful or refer to something in a later area. Once you get to that area, there is no indication that you needed to use a clue from earlier.
-There are several points in the game where no clues are given for progression. Several times it is necessary to walk through illusionary walls, or break floors/walls to continue, with no indication you need to do so. You are left jumping around constantly attacking everything in the hope you will accidentally solve a 'puzzle'.
-The above point wouldn't be a problem if the game didn't frequently PUNISH you for trial and error. Yes, there are many rooms in the game where, if you attack random walls, lightning strikes your character and does a substantial amount of damage. The game teaches you quickly not to attack random things, but then at other times the only way to continue is to know where to do it. I have died many times, just attacking random walls to see if they would crumble, only to find out that I was indeed supposed to attack a specific part of a wall; but no indication was given to do so.
-Unfair instant death traps and rooms that you simply become 'stuck' in. There are many rooms that just kill you, and other ones that just trap you, forcing you to restart from your last save if you don't have specific items.
-Some puzzles don't reset. Make a mistake and you can never get the item. There are only a few of these, but there isn't any indication that the puzzle will be like this. Once again, the game punishes trial and error. Fortunately, I think they only do this with non-essential items.
-The game doesn't control well enough for some of the fast-paced action segments they throw at you. Boss battles typically aren't hard for any reason other than your character moves slowly and doesn't control in the air well.
If anyone ever tells you that they beat this game without ever consulting a guide or asking for help, they are lying. I really wanted to like this game; thematically it is amazing, and I love this style of metroidvania action-adventure. I find myself being forced to use a guide all too often, and it really ruins the gaming experience (seriously, some of these 'puzzles' are just impossible unless you know exactly what to do beforehand). If you really like games like castlevania 2, or don't mind having a guide open during your playthrough, then La Mulana might be for you. Otherwise, stay away.