As for level 9 giant beasts that sneak attack from hiding, they are rarely seen because their bodies have almost completely transformed and no longer need to hunt for food.
When mutated creatures reach level 7, their bodies begin to undergo qualitative changes under the influence of their gifted powers, like the metal giant beast Jack Clark had once encountered.
Likewise, creatures like the Single-horned Tiger Whale and the Deep Sea Dragon Turtle actually have very little dependency on the flesh and blood of mutated creatures.
More often, they absorb energy from between heaven and earth to maintain their consumption and purify the growth of their bloodlines.
After reaching level 8, the physical qualitative transformation becomes even more obvious, with the flesh and blood of fire giant beasts mostly crystallizing into magma, and the flesh and blood of those with rock-related talents turning into what could be described as Vajra rocks.