If you get a special chest like Holy Chest or Ice Chest here is where I have found to hunt monsters in the most efficient manor.
Below are the levels that have all one color/element. Most have 11 monsters of the same color which makes getting 150 monsters pretty easy.
==Season 1==
PURPLE or Dark
3.4, 4.3, 5.5, 8.4, 10.4, 11.7 and 12.5
GREEN or Nature
6.3, 6.6, 7.5 and 17.5
BLUE or Ice
8.1, 8.3, 8.5, 9.1, 9.3, 9.5, 13.5 and 13.8
YELLOW of Holy
9.4, 10.6, 12.3 and 16.3
RED or Fire
19.4, 19.6, 20.4 and 20.7
==Season 1 ( includes reds for lower level players )==
Rook's List
==Season 2 ( currently province 1- province 6 are open )==
RED or Fire
3.4, 3.8
Season 1 ( 4-1 works very well also for FIRE)
Mod Edit:
@sommc’s guide about these elemental chests: