how do you enter that blocked cave at the elf village?
If you mean the cave to the right of the village then you get information from talking to the some elf.
You get the information that you need to have the "water of transformation" in order to enter the cave.
If you try to enter the cave after getting that information and you have the "water of transformation" then you will be able to enter the cave using it.
However unless you use a save editor to cheat to get it you will have to work quite a bit to get it.
The "water of transformation" is obtained in the Trial Dungeon.
I do not remember where exactly it is but it is definetely beyond floor 42 of the trial dungeon (so floor 43-50).
Maybe you only get it by beating the boss on floor 50 - I do not remember exactly.
Just one tip:
There is a floor with purple ground in the early floors of the dungeon where you can use empty bottles to collect "deadly poison".
You might want to collect as many as you can because this can help immensely in boss battles with huge amount of HP or/and regeneration abilities.