The Goddess of War Bellona led the charge towards the Primordial Beast of Sloth. The Primordial Beast of Sloth is not a primordial beast that could be said to be very active even when it is attacking. This led to the army to think that the Primordial Beast of Sloth is severely weakened and will not retaliate as much.
The Primordial Beast of Sloth is literally a large sponge for damage as all it did was stand there and endure the attacks. It would try to attack via an energy bomb ten minutes or so, but the movement of the Primordial Beast of Sloth is slow enough that most of the people have already time to evade. The large health pool of the Primordial Beast of Sloth started to slowly drop down as the number of people attacking it has increased.
