I posted a little bit back, the only thing I can think of for full clear is that you complete goal 3 on every expedition. If you check the data that the Burrow NPCs store, they have info on which goals you completed from each expedition, independent from the grimoire you have traded (you can even check this when no grimoire is traded), so maybe that's what it is. Makes no sense that it would just be clearing all expeditions, because just killing Umagrhk (researcher emeritus) would be the same thing.
To clarify Seiji's post... on one set of runs, you use Diligence as the "progress" book and unlock access to the bonus to spoils expedition separately on the Avidity book. Then, you do Researcher 4 with 1/6 using Diligence and 5/6 using Avidity. Finally, everyone does the boss run with Diligence. The one diligence grimoire with the "bonus to spoils" will give everyone bonus to spoils, and everyone else will have bonus to spoils still on their "Avidity."
Next set of runs, 5/6 people use Diligence as their progress book, and 1/6 people (one of the people who still has bonus to spoils) use Avidity as their progress book. Now you don't need to redo Researcher 4 to get buffs again because you already have them recorded. You can repeat for three more sets of runs/bosses, with a different person using Avidity each time. Finally, when you're down to just 1/6 people left with bonus to spoils, you have everyone redo Researcher 4 to get bonus to spoils back for everyone on Avidity. Wash, rinse, repeat. In this way you only need to do the bonus to spoils run (or any other buff-gathering run you choose) on 1/6 sets of boss runs. Obviously this is all assuming six people, would work for less too, just slightly less efficient.
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