The phrase "assiduously prepare" is correct and usable in written English.
It can be used when describing a diligent and persistent effort in preparing for a task or event.
Example: "In order to succeed in the competition, the team must assiduously prepare for every challenge they might face."
Alternatives: "diligently prepare" or "meticulously prepare".
Dunst assiduously prepares for her roles with what she calls "dream work".
For Mr. Osborne, 39 — the youngest British chancellor since 1886 — it was a defining moment which he has been assiduously preparing for since 2008.
For Mr. Osborne, 39 -- the youngest British chancellor since 1886 -- it was a defining moment that he had been assiduously preparing for since 2008.
Mr. McCain has been assiduously preparing for this week in New Hampshire, presenting himself as both the political reformer, an appeal that could draw independent voters, and the more responsible steward of the nation's foreign policy.
Though he blames the government for his company's downfall ("We got dealt a bad hand"), he began assiduously preparing for this month's changes in rehab hospital reimbursement.
On more occasions than I care to remember, I didn't close the lid properly and while assiduously preparing for the next step/sneaking a quick cup of coffee (delete as appropriate), the centrifuge failed to start.
De La Rue worked assiduously to prepare the latter for operations, as it was the only fighter squadron able to defend Perth and Fremantle.
Both author and producer assiduously revised, prepared and rehearsed every line, scene and setting in the months before the premiere, creating a carefully constructed representation of late-Victorian society, yet simultaneously mocking it.
Mr. Greenspan has assiduously sought to prepare investors for a return to higher rates, making a series of minute changes in the wording of Fed statements to signal that a change in policy was gradually drawing near.
