How you obtain these elements is by going to the item dungeon NPC and selecting element dungeon there are 3 different ones but all are fairly easy depending on your lvl i would recommend lvl 40-50 to solo up to the 1st one unless you are in a good party 50-60 for the stage 2 and 60-70 for the third ( I'm lvl 80 gl and still sometimes leave with half to a fourth of my hp gone out of 7k) and of course offline dungeon gives the best drops and for me it seems that only the stage 2 dungeon will drop light and dark fragments but I will discuss those in a minute. It takes 20 fragments of that element and a none bowl to make a element bowl (to get a none bowl you need 15 none fragments also obtained from element dung (I have also heard you can get one from a boss but I don't think that is true so I wouldn't try it unless you wanna prove me wrong ^^)).
Various monsters have certain elemental attributes to them and having the element that is strong against it will offer more damage in the new update coming out it is indicated that the monsters will have their element next to their name. This goes for the defense as well though monster attack you with a element that is weak against your element type then it will do less damage. I think this goes without saying but if the monster you are fighting is strong against the element that you have then you will receive greater damage to you.
So now that the monsters are out the way lets talk about pvp, to make all this short apply what I said above and boom same thing only problem is like not knowing the monsters element you don't know their element so hope you get the luck of the draw there. By now your wondering all this and I still don't know about light and dark fragments well here we go. From what I can tell these are the two best elements 1 because they are hard to get and 2 because they don't apply to the other elements only light and dark.
*Light VS Dark will increase each other’s damage, and same elements each will decrease more damage.
Now some of you are like what does that mean I was right there with you lol but basically it means the strong will survive meaning they give additional damage to all the elements by a small percentage but the most is from light v dark. So in other words light v light or dark v dark = no damage bonus, light v dark = additional damage, light or dark v any other element= small additional damage. Basically light and dark elements are mainly for pvp.
Another thing to mention is when you put a element in a weapon or armor you can not evolve it anymore except by removing the element with a none bowl and then you will have to make a whole new element bowl so don't waste your light or dark bowl on for instance a 1* weapon pr armor cause that's a waste so I will say this again once you use a none bowl on a element so you can evolve it you will NOT get your element bowl back and you will have to remake a new one.
I hope this guide helped and PM me if anyone needs a guide made on something I will see if I can do it... Don't forget to like if you found it helpful (side note if you are a guest or a robot join the forums there's plenty of things you can learn here and sign up is super easy takes like 5 minutes or less).
EDIT:Well turns out the attacks with elements in they name do have an effect on that type of monster eg. I used flame burst on water mob with non-element and earth elements around and the damage done to it was less than the damage to the others.
EDIT2: If you have a element in a weapon and the attack has its own elemental effect the attacks effect cancels out the effect on your weapon.
EDIT3: Now you can evolve and keep your element in your weapon.