I always take a selection of weapons irrespective of Burn modifier, arc for fallen captains, void for Vex minotaurs, etc. to get down shields before using the burn weapon. "It seems that Bungie applies the 300% modifier FIRST and then the elemental shield reduces that number by 200% due to the non-corresponding elemental damage. This would make the modifier 150% for the burn element to the non-corresponding shield element. In conclusion, it would be smart to assess the weekly nightfall to see what shield types there are, then select weapons accordingly to take down enemies the fastest. For example, if most of the enemies have void shield, yet the burn element is arc, take a void weapon to take down the shields then use the burn element to finish them off. This is smartest in a scenario where you're taking down an ultra or heavy mob." Source: http://www.reddit.com/r/DestinyTheGame/comments/2hypnc/elemental_damage_nightfall_burn_damage/