Trample
When Phyrexian Dreadnought enters the battlefield, sacrifice it unless you sacrifice any number of creatures with total power 12 or greater.
If you are unable to sacrifice 12 power worth of creatures, then you cannot choose that option; in that case, you must sacrifice Phyrexian Dreadnought.
If the Dreadnought is no longer on the battlefield when the triggered ability resolves, you may still choose either option.
Reverted to its original wording, this now has an enters-the-battlefield triggered ability. During resolution of the triggered ability, you choose one option or the other.
Phasing in does not trigger “enters the battlefield” abilities, so you don’t have to sacrifice again if it phases in.
The sacrifices are simultaneous, not one at a time.