Say what you searched, and what you did not
A statement that lists custodians, date ranges, sources, keywords and the reasoning behind each choice is difficult to attack. One that says a reasonable and proportionate search was carried out invites an application.
Being open about what was excluded, and why, is stronger than implying nothing was.
Show the numbers
Volumes at collection, after deduplication, after filtering and after review tell the court that the exercise was managed. They also make a proportionality argument concrete rather than rhetorical.
Keep the chain of custody boring
Dates, hashes, names and locations. If the chain of custody record is dull and complete, nobody spends a hearing on it, which is exactly the outcome you want.