The skill’s description says that the skill scales with atk and enemy max hp.

The problem is that this skill does the same damage to easy and normal clan boss(aroud 100k critical or around 130k if septimus passive procs) when normal clan boss has more than 3 times the easy clan boss’s Max hp.

Increasing septimus attack trough gear or buff (25% or 50%, tested both) wont increase the damage.

Increasing septimus critical damage wont increase the damage.

Decreasing boss defense by 30% or 60% wont increase the damage. (Tested both).