Boss kills. Make sure to use elixirs/incense/dungeon xp bonuses. However, just lair bosses this season. It was far more important for the devs to make sure to have the DEI hire on the campfire, than put in any attention to proper balance of xp.

What I started doing was setting up a group, 1 person starts the boss, another kills, another opens the box. Do not have all 4 people opening the box. It's about getting more boss spawns, not about the loot from the box {which is usually ass anyways}.