The question is unclear.
The verbal phrases "do so" and "did so" always refer to an antecedent verb. The "so" is seldom, if ever, required grammaticaly, but it may be used in certain circumstances, e.g., to give emphasis. I do not agree that it neccesarily indicates reluctance.
When, after so many years, we were finally free to marry, we did so immediately
is not to be read as an unwelcome marriage.
It is particularly helpful to use "so" when the antecedent verb referred to is qualified and it is intended to indicate that the qualifications will be or have been met.
He suggested that we order the needed parts from a supplier who could guarantee consistent quality and prompt delivery. We did so.
In the example above, the "did so" indicates that not only did we order the needed parts, but we ordered them from a supplier of the type suggested.
Your example related to "fierce" is not grammatical. The use of "so" with adjectives (and adverbs) is a bit complex.
He was as fierce as the warriors
is a comparative indicating equivalence. He is neither more nor less fierce than the warriors.
When negating such an equivalence, many would use, and some would require the use of, "so":
He was not so fierce as the warriors
He was less fierce than the warriors.
Furthermore, "so" can indicate a causative relation
He was so fierce that the warriors fled.