If you do a world event you get a chest - elites/legendarys will spawn in world events and world events will defo spawn when in free play. It's not RNG when it will happen very easily by just playing. And most of the tasks are killing enemies using your different skills anyway.
Keep in mind, that every player needs to open 15 chests, not just a player of the group or your friend, that you play the game with (co-op game... and it forces you to do free play in public groups, solo is not an option).
Yes, this will get fixed, but for example my friend opened most of the chests, so he only had 3 chests left on the quest, while I needed 14 ;) So, it was a bit annoying. But for me the quest itself is well bad in itself, not due to the (minor) grind.
You should prove worthy to enter the tombs, once you finished those grindy steps for the quest, it goes: Enter the room, which looks the same for every tomb, get a loading screen before it, walk some steps straight, click on the coffin, get random loot, get out with another loading screen. Repeat that step 3 more times, what an epic journey ...
