When you kill a Mongol leader, the counter pops up to say observe or kill Mongol leaders to advance. I have never observed any. I am at a Mongol camp now and one of the objectives is to observe the Mongol leader. I can see him down at the beach practising with a spear but I don’t know how to observe him.
Observing Mongol leaders is similar to surveying camps which you should have done a few times by now. To observe the leaders, you will need to find a high up area, usually in the mountains nearby the camp where you have a clear view of the leader.
I think I know the camp you are at now. If you look near the beach, there is a sort of rock feature that arches over the camp. You need to climb up the mountain and run across this arch until you find long grass overlooking the beach. You should see the prompt here to observe this leader. You can look out in other camps for similar opportunities to get a look at the leader before you kill him.
Theres more than enough leaders out there to get the upgrades. You dont need to observe any to max this out so there is not need to worry about it too much if it becomes a chore.