Since the boss stats were significantly better in defense and attack, he was heading toward the twins quite slowly. It wasn't as slow as the slime bosses when they were slowed down by Athis and a muddy ground, but it was still slow enough for the boys to not be scared at all by the boss.
A monster that couldn't touch its prey wasn't really scary after all, so the boys weren't that afraid. However, it was when people were underestimating an enemy that they were losing their lives. So even though they weren't afraid, their guard was still up. A mistake would be disastrous since they will just lose their lives immediately, so they needed to prepare for every single problem that could arise during this fight.
So since the boss was slowly moving toward them, the boys too began to make their moves.
