Bosses in Crystal Ore

Is there any way to tell which of the three (?) Bosses in the Crystal Ore map is vulnerable? Maybe I understand it wrong but I think that only one of them is vulnerable at a time.

Recently I die a lot to these bosses and I cannot figure out what to do.

Any help would be appreciated.

There's a marking at the feet of the one who's vulnerable.

Best way is to have massive AOE or gather them all in one place, realistically you don't have any chance of seeing that small yellow circle art their feet with everything going on on screen. Next best thing is trial and error, attack one, see if he takes damage, if not move on.

Thank you very much!

Maybe I was just luckier some patches ago, but I remember that these dudes used to gather together in the centre of the area. So I somehow always managed to hit the right one without taking too much care. Now at least one of them prefers to wander around in the arena.

I'll try to see marking the next time I'll be there. Somehow I like the map. I'm just glad that this boss mechanic seems to be rather unique.