To kill a thief you must see that thief coming or you will have no chance to react in time. My best advice is look around and identify the thief early. Watch for them to close to 800. Be ready to counter or not be where they think you will be. You can build tanky with auto defenses if you want to but that just buys you a little time and in the end you won't have the dps to kill regular players. Run with a small group and communicate, run with a big group and communicate. Or just land [Moa ](
)a one second cast time with an audio and visual queue as big as half your character and get lucky enough to land it and do enough damage to kill them while most people around you don't react fast enough to help. Also give up your most powerful cooldown for a 1 shot attempt to kill just about anything.