Shelltron and critical attacks.

[Machka]

Errrr Nope, they don't crit, atleast not unless its a specific mechanic. I'm 99.9% sure on that, but feel free to provide a source saying otherwise.

Raid boss autoattacks absolutely can crit. It's possible not every boss does, but there are definitely some that do.

Source: Used to raid as a tank, and have 100% definitely been critted by autoattacks before.