Two things to succced against Hostile Forces

There are two things that make it impossible for them [the hostile forces] to succeed even temporarily in any attack on the mind or the vital—first, an entire love, devotion and confidence that nothing can shake, secondly, a calm and equality in the vital as well as in the mind which has become the fundamental character of the inner nature. Suggestions then may still come, things go wrong outside, but the being remains invulnerable. Either of these two things is sufficient in itself—and in proportion as they grow, even the existence of the hostile forces becomes less and less of a phenomenon of the inner life—though they may still be there in the outer atmosphere.

Ref : Letters on Yoga Vol. III