To get the most out of your boss clears, you need more people alive!
To ensure this, try to get everyone with +10 purples minimum, make sure you have a surplus of luck potions available. Each unit that is present when the boss dies, will get loot.
Dead units, hungry, tired or moody bastards that leave, will not stick around for loot and will run to town. Thus, not getting anything.
If you are struggling for essences, because only 1 of your units is clearing the boss, this is why. You are only generating 1 unit worth of loot.