The DLC first boss's 16.54% does not measure difficulty: it measures who bought it
The expansion's three story bosses lose half a point between them: whoever starts, finishes. Then the new trials open, and that is where the percentages drop below ten per cent.
And it counts towards A Life Dedicated to Duty, The Two Masters, Scornful Mother of the Damned, More Machine than Fiend, required for the platinum.
Three bosses, half a point apart
The Quest of Duty — 16.54%. Defeat the mysterious demon in Tokyo, the first story boss of the Two Masters expansion.
That number does not measure difficulty: it measures how many people bought the DLC. The expansion's eight trophies sit apart from the base game's platinum, so they are out of reach for everyone who does not own it.
If you arrive fresh from finishing the game, the expansion assumes the entire system: it reintroduces nothing and opens at the level of demand where you left off.
A Life Given to Duty — 15.91%. Defeat the new threat from the shadows, the second story boss.
The drop from the first boss is just six tenths: whoever starts the expansion finishes it.
This is the point to decide whether you are going for The Way of the New Master Ninja, which asks you to redo the whole expansion on Master Ninja.
The Two Masters — 15.21%. Bring peace back to Tokyo once more: the expansion's ending, half a point below the previous boss.
Finishing it opens the rest of the content: the new boss trials, the EX trials and the Abyssal Path mode, which are the five remaining trophies and the rarest on the whole list.
The two boss trials that fall below ten per cent
The Scornful Mother of the Damned — 8.46%. Complete Seere's boss trial, one of the new trials the expansion adds.
Seere appears in two distinct versions among the expansion's trials, one built around speed and one around power. Clearing the one the trophy asks for is enough: you do not need both.
It sits below ten per cent, already in the range of trophies almost nobody holds. In return, the expansion's boss trials each pay over twenty thousand ninja credits.
More Machine Than Demon — 8.75%. Complete the mechanical soldier's boss trial, another of the new trials.
It is practically the same percentage as Seere's trial: whoever does one does the other. There is no wall-and-formality pairing here; it is the same barrier seen twice.
Like the other boss trials, they can be repeated without penalty and they pay well. If you are going for The Way of the New Master Ninja, the credits they leave here fund the items you will need later: repeating these two trials is the fastest way to fill the crimson blood inventory before the Master Ninja run.
From the three story bosses at 15-16% to these two trials at 8%, the expansion loses half of everyone who bought it right as the story ends. The content that follows is more demanding than the campaign preceding it, and that is where 100% is decided.
The expansion's boss trials repeat without penalty and pay over twenty thousand ninja credits each: they are the fastest way to fund the Master Ninja run.
The trophies involved
Each one links to its section inside the full Ninja Gaiden 4 trophy guide.