Assuming it appears only once in the document, and each line of your assessment is terminated with a paragraph break and not a line break then
Code:
Dim orng As Range
Set orng = ActiveDocument.Range
With orng
.Start = .Start + InStr(orng, "Assessment")
.MoveStart wdParagraph
.End = .Start + InStr(orng, "Diagnosis") - 2
.Style = "List Number"
End With
Set orng = Nothing