As everyone else has pointed out it is based on your contribution (emnity). If he was killed quickly with a lot of people fighting him. “Your” contribution had also a shorter window to get credit, even if you were there for the “whole” fate. 10 minutes of effort is much less than 2. If you want to roll solo I would suggest getting a Stygian ash and using it on him to buff your contribution and spam emnity combos as a tank. (The ash is a tip I heard of seems to work but not 100%)