Most of the security was not based on the protection from the teachers. It was based on Hogwarts Castle being secure.
Hogwarts security is mentioned multiple times in the book - you can't apparate there, you can't even reach it on a broom (in the end of book 6, <PERSON> has to remove protecting spells to reach the tower). The Death Eaters couldn't get there without the vanishing cabinet.
The protection from the teachers was just an additional level of security and it was there because of the plot.
The security was silly to begin with:
1. Fluffy - an animal, probably possible to be killed by magic. If not, it has to sleep sometimes. Using an invisibility cloak or some other magic could probably enable any wizard to pass it unnoticed. If not, evil wizard have lots of options, for example, taking someone else there and using them to lure the beast to the other side of the room. Also, someone has to feed Fluffy. Listening at the door how that one is dealing with Fluffy when bringing the food is the most simple way to learn about that music stuff. Another option would be to study books about handling this kind of animals.
2. Devil Snare - something that everybody learns in school in their first year, so probably something pretty basic. The chance a potential attacker will know it is great. If not, fire is probably the first thing they would try. Or, they would be cautious enough not to jump blindly into nothingness - they could fly down creating light with their wand, for example. This would work much better by removing the Devil Snare - killing buglars instantly by the long fall and hitting the floor.
3. Flying Keys - some powerful magic could probably attract the key or just break the door. Note that <PERSON> was able to get past this pretty quickly, so he had to have another key or he just used a more powerful magic. 300
Why having the keys there at all? Why not just wear the key in your pocket instead? This is just making it easy for the buglers.
4. Chess - why Chess? Why not an army of Chess pieces attacking anybody entering the room? Why the chess pieces were not a bit more clever? If a 14 years old boy could defeat them, what's the point? Again, <PERSON> was able to get past this room quickly, so he either smashed the pieces by magic (in the beginning of book 6 <PERSON> can apparently destroy whole bridges), or he had some kind of backdoor. Allowing a burglar to pass through this room just by winning a chess game is a huge security breach.
5. Troll - worse than Fluffy. Can be knocked out easily so also killable by magic easily. A very difficult security measure because it has to be fed. That means that someone has to go there every day and feed it (note that food cannot be created by magic). The interesting idea is whether they made the troll to learn <PERSON>'s smell or whether they just knocked out the troll everytime they went to check on the stone.
6. A very simple logic puzzle. Again, the question is why giving hints at all. Why not just keep the correct vial in you pocket and remove everything else from this room, keeping only the fires.
7. Mirror of Erised - a psychological protection. <PERSON> found the stone because he wanted to find it. <PERSON> didn't find it because he wanted to give it to <PERSON>. Frankly, this is more based on a play with words than on psychology. I don't think this is a good protection and there are too many questions involved. For example: would breaking the mirror destroy the stone? Where was the stone actually stored?
On a side note, a more direct approach would be to find <PERSON> and beat the hell out of him to get a vial of the Elixir. 300