Phantoms and monsters became stronger in many ways, each with many opportunities to increase their power.
It could be new abilities, long training, experience, improved skill in their weapons, or even an aura.
But, in its purest form, everything started and ended with one crucial thing - energy, density, and quantity.
It didn't matter how many different abilities, experience, and weapons a Phantom or monster had if someone like the Dormant Lord would kill them in the blink of an eye. Dozens of attacks from weaker creatures wouldn't even leave a scratch on his body.
It was as pointless as trying to break a steel beam over and over by attacking it with a simple branch fallen from a tree. It would be more likely for the branch to shatter into splinters and the attacker's hand to be wiped to bloody dust than for the iron beam to receive any visible damage.