There are two things we need to do in order to attain enlightenment: purify our negative karma and obscurations; and accumulate merit and wisdom. Both are required. The best method for purification is the sincere practice of Vajrasattva, accompanied by the Four Opponent Powers. “The Preliminary Practices of the Incomparable Drikung Kagyu” states:
Generally speaking, virtuous and non-virtuous acts depend on mind. The mind is the master and the body and speech are the servants. Because of the degree of the power of afflictive emotions…we create non-virtuous acts (which will) bring suffering and undesirable conditions…Therefore, it is very wise to purify all non-virtues we create before they bring the result. This purification…can be very effective…Vajrasattva meditation is a special practice for the purification of non-virtuous causes, so with confidence and without hesitation, enter into the Vajrasattva practice.
In The Words of My Perfect Teacher it says:
The main obstacles that prevent all the extraordinary experiences and realizations of the profound path from arising are negative actions, obscurations and habitual tendencies. Just as it is important to clean the surface of a mirror if forms are to be reflected in it, so too it is important to eliminate our obscurations so that realization can appear… The Conqueror taught countless methods of purification for this purpose, but the best of them all is meditation and recitation related to the teacher as Vajrasattva.