Tiraba lenta

Tiraba lenta
 

Hi,

Entonces, mientras tiraba lenta pero firmemente del brazo, hizo presión con el
pie.

It looks like a case of agreement in gender, but totally impossible. If not a typo, what is it?

It's not tiraba lenta, it's tiraba lenta(mente) pero firmemente.

When using a series of 2 or more adverbs that end with the suffix -mente to modify the same thing, one drops all of the -mente suffixes except the one on the last adverb that ends in -mente in the series. Why the feminine form? Because the -mente suffix that one adds to an adjective to derive an adverb comes from the feminine noun mente, and the adjective modified it. Why drop all of the -mente suffixes except the last one in a series of adverbs? Because repeating -mente with every adverb in a series sounds bad.