There is a cooldown if you are killing the same boss over and over thats why I did these 4 on rotation so hopefully the cooldown is gone by the time I get back to the repeat. Was looking for a 5th to add in but havent found an easy one yet. Bracus, prohibitive mind and sword of crota ultras dont count as ultras. I know the sol progeny bosses count but too long a mission to farm.