User blog comment:B214/Volume 22 spoilers/@comment-30048084-20161031183100/@comment-29963364-20161031221405

About what Lady Phenex said about Ravel following the "Path of Supremecy", and how Issei should be careful with her...

Following the "Path of Supremecy" means to gain power through military/combat prowess with the goal of outright subjugating any & all competition or resistance.

This conflicts with Issei who follows the "Path of Rightiousness."

A good way to describe the two "paths" is to compare the ways Issei and Vali both obtained their [Cardinal Crimson Promotion] and [Empireo Juggernaut Overdrive] respectively.

Issei: He gained the CCP by persuading the past possessors through his perseverance and his desire to protect his friends and those he loves. Unlike the past possessors who seeked power to dominate and/or destroy their enemies, Issei only seeks power to save and/or protect his loved ones. Issei's "rightiousness" inspired the past possessors and they placed their faith in him.

Vali: He gained the EJOD by doing what the past possessors did (seek power to dominate & destroy their enemies), but to the nth degree. Vali essentially "overpowered" the past possessors through his passionate desire for combat. They gave into Vali's "supremecy" and surrendered. In a way, he beat the [Juggernaut Drive] into submission. Read the LN, vol.12 when Vali first activates the EJOD against Pluto, and pay attention to what the past possessors say as Vali recites the chant.

The reason Lady Phenex tells Issei to be careful with how he "uses" Ravel is likely because, since Ravel is Issei's manager and stategist and in charge of progressing his political career, Issei should be careful that Ravel doesn't take action or use methods that conflict with what Issei believes and is striving for. Ravels skills as a follower of the "Path of Supremecy" will definitely be useful, but Issei should find a way to make those skills work for him.

At least, this is all my opinion/speculation.