I've done a few of these events, and generally (if they're champion) you need a least 4 people to even ensure their shields aren't constantly recharging. Even if you have a lot of people, he just makes a beeline for the exit once you start hurting him. So far the only one I've completed is one where we had multiple titans constantly punching him on and off. which I'm not sure this is the intended way to win.
-
The runners are undoubtedly the hardest events, especially with how quickly the shields restore. That being said, I've never come across one that can't be beaten by 2 players in the mids 20s or higher. They aren't that bad.