It's fiercest. Adjectives with only 1 syllable (like fierce, or big, or small...) take an -er to make them into comparatives. The exceptions are 'good' and 'bad', which change into different words. Most words with 2+ syllables need 'more' or 'less', apart from words like easy, which end with a 'y' - words like that become 'easier' - you take away the Y and add -ier :)

I would say fiercest and guard dog

And I would say "guard dog".

I think it's "fiercest" but if you said "most fierce" it's perfectly okay. (I'm finding conflicting information on the internet but I believe both are okay).