This type of mission will occasionally appear. You will need to defeat 150 guardians of designated colour.
These are the provinces where you can easily attack more fire monsters and win the chest as quickly as possible. They are available in other provinces but only appear in 1 stage so will take longer time if you wish to attack, but then still waste world energy and lower number of recruits obtained.
These provinces here are much more difficult but there are fire guardians waiting at all waves.
So if you have lots of loot tickets, you might want to consider using them in this event. Or perhaps place one more hero of ice element to double (or triple if you place 3) the damage.
goodluck
