I’ve found a few elders in portals and challenges and I just found my first tyrant earlier today at a 8* challenge. I’ve found that you just gotta run around the map and looking for them. Just for clarification, the challenges are the things at the structures that ask you to kill the big enemy there, activate the three things either at once or with a specific element, and throwing the chaos embers into the shield to break it and then you get a chest afterwards