Eren's diligent work in trying to accommodate various ways of fighting was paying off.
His inquisitive nature and his will to take risks using his opponents as test materials had allowed him to broaden his horizons so many times at this point. This was the reason why he could use the class of an assassin so flawlessly.
Eren also realized that Life Drain was only this potent because he was using it inside the Domain of Gluttony. He could use the gluttony mana that was all around him to break conventional logic and kill his opponents this way. However, he would be unable to repeat this feat easily if the domain wasn't present.
It would also be possible for him to do the same without having to deploy the domain if he had enough mastery in regularly harnessing gluttony mana. However, he needed to fine-tune his mastery of the Sin of Gluttony for that to happen.
