What is the comparative form of word fiercely?

The word "fiercely" is already in the adverb form, meaning it describes how an action is performed. It doesn't have a comparative or superlative form in the same way that adjectives do.

To express a higher degree of fierceness, you would use adverbs like:

* More fiercely

* Even more fiercely

* Much more fiercely

* Extremely fiercely

You could also use other adverbs to convey a similar meaning, such as intensely, aggressively, vigorously, etc.