There is nothing but luck involved in getting drops. Luck elixer (and the luck amulet) will increase chances, but that's it. Just be there when the boss dies and you might get something.

Locked chests drop from anywhere, not just bosses. Kraken Mines 3 is the best bet for crates, bosses do have a higher chance to drop crates and KM3 has 3 bosses that take about 2min (average for decent group, or solo) to kill.