A short field season creates pressure that immigration steps do not respond to. Documents, credential evidence and legal review proceed at their own pace, so the file should be assembled well before the season opens rather than in the weeks preceding mobilization.
Build the file ahead of the season, not into it
Work back from the first day the applicant must be on site and identify what has to be finished before then, including the employer letter and any credential documents from institutions abroad. If the season is likely to arrive first, ask the employer whether the program can begin with other staff, or whether desk-based preparation can proceed from Canada while the file is completed. Hypothetical example: a geotechnical engineer must be on site the week the ground thaws, a date nobody controls.
Working back from that week usually reveals that the longest item is not the immigration step at all but a transcript or a credential evaluation from an institution abroad, which can take weeks and cannot be accelerated by explaining a field season to anyone. Order those first, and treat the employer letter as the item that closes the file rather than the one that opens it. Build a written sequence with dates: documents requested, documents received, letter drafted, counsel's review, earliest realistic presentation.
If the sequence shows the season arriving first, say so to the employer early, while the program can still be re-staffed. A file assembled under mobilization pressure tends to contain the errors that take longest to correct.