Arcana obscura under 8 mins?
Yes, it’s possible. For the shards I think I distracted things with illusions, then stealthed using MI + decoy while standing at the crystal to give me enough time… Only stealthed 2 of the crystals to allow for MI recharge. there’s also probably room to burn things down faster or capture the throne more quickly when getting the mobs off it.
Is it possible on mesmer? Yes. Is it possible the teapot orbiting the solar system spontaneously decides to head straight for my coffee table burning up in the atmosphere and land as a small bit of dust? Yep also possible.
Generally I found the entirety of season 2 plain old easier on any class that didn’t use a pet or rely on AI. The easiest to do what you’re asking was thief as stealth wins most stuff. I attempted a few things on Mesmer but decided it was just easier to get achievements on anything but a Mesmer.