and of course optimization for phones
1. To access the cave, you can get an empty lantern on the beach and fuse it with light crystal ( I remember these are in silver chest).
2. Some monster can cum, but you should level up your character to get more spell (charm) and skill (battle menu or battle details). After you have those , you will notice that some enemy have white bar. Use the spell I refered above to increase the white bar, then you will see what happen. This is one of some secrets mechanics. Deimus himself also have talk about it in some comment in the past.
I think you should play story more, because everything in this game is connected, try to look the quest button on the right, it have a lot of hint ( and the barkeeper too ).