had been mastering
-había estado dominando
Past perfect progressiveconjugation ofmaster.There are other translations for this conjugation.

master

For many years the young man had been mastering the stages of the Path under the guidance of the Immortal Master.
Muchos años tuvo que estar el joven adquiriendo maestría en las etapas del Camino bajo la guía del Maestro Inmortal.
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