The word which can be found in a comparative degree would be stronger since it comes from the adjective strong, and it has the ending -er that corresponds to the comparatives structure. There is also the adjective heavy, but in contrast to stronger, heaviest is in the superlative form, because it has -est at the end. On the other hand, there is easily that is an adverb, so in this case, the category is completely different. Finally, regarding fierce, this is just and adjective without any modification, therefore, it is not a comparative.
