I agree with you Fencers that the largest world DEs will not be epicly hard...because Anet want ppl to experience them and down them. You're likely right in that they will be co-op obective based, and that it's doing those right rather, or at least as well as, the standard mechanics of a big world DE boss that will ultimately lead to whether it's downed or not. Ur laser example illustrates that quite well.
But who knows rly at this stage.
I also expect the larger DEs to not be done very often by ppl. It's more of a part of the story rather than a do it x number of times. There's no real reason to do these epic repeatedly ones unless u really enjoy the fight. We also don't know how often they'll happen, they could be part of the ones that happen every month of so.
Personally, I wish Anet would hurry up and show us some more of the dungeons. That's where they've said the hardest content will be. It's of course really hard to judge just how hard an encounter is just by observing it...but showing the explorables would help in showing ppl that there really is very hard content (in theory ofc lol).