The Kaiju in the world of ATOM can regenerate, but how much damage does a Yamaneon infused monster need to sustain in order to die?
A hell of a lot. The Terror got his legs and stomach caught in a landslide, where they were crushed beyond use, but managed to hang on for a while (albeit with some help from reverse engineered alien healing technology), and Ahuul manages to survive being immolated.
Although it’s really less a matter of quantity of damage and more about quality - specifically, how vital were the organs that got damaged? Destroying the brain is a surefire way to kill a kaiju in most circumstances (a large burst of radiation can put the healing process into overdrive to compensate for even this level of destruction). The hearts and lungs are slightly less vital - a kaiju can regenerate them, but if you keep sustained fire on them it may take too long for them to heal and cause death. Finally, targeting the digestive tract will slow down their healing ability some - while Yamaneon can (impossibly) “create” matter on its own to heal a kaiju, it prefers to instead draw that matter from natural processes like digestion. It’s easier to convert food into flesh than to do the (normally impossible) feat of making flesh from thin air.
There also seems to be a psychological effect to it - again, the poorly hidden secret of how kaiju work in my setting is the fact that Yamaneon is an intelligent entity that has a will of its own. It subconsciously communicates with its hosts, and as such a kaiju’s Yamaneon powers are in part determined by how the kaiju perceives itself. If a kaiju thinks it can fly, then Yamaneon lets it fly, and so on.
This means that is a kaiju thinks “well, this should kill me,” Yamaneon will let it die and eventually convert its mass into more Yamaneon. But if a kaiju is determined to press on - if it refuses to let its damage take it down - Yamaneon will do all it can to honor that wish. A determined kaiju is almost invincible.