The slipperiness of the monsters in Rocklands is much higher than in other places.
Before the Conqueror Army arrived, they already received the message. Any Monster Tribe with more than a hundred members either desperately drilled into the Stormhorn Mountains to the south or retreated to the vicinity of the Enorok Great Desert to the north.
They are adopting a posture of 'chase me, and I'll go into the deep mountains or desert to play guerilla warfare with you and see who can hold out longer.'
Having powerful Strength means nothing if the enemy refuses to engage you.
The Conqueror Army achieved magnificent victories along the way because they've always played the role of the weak, at least in the eyes of those Monster Forces. In their own perspective, they possess the Strength to swallow them whole.
However, they kicked a hard board, broke their claws, and allowed the Conqueror Army to swallow them whole, becoming the nutrients for their robust growth.