There are a variety of synonyms for the phrase "in a cruel manner." Some common options include "viciously," "brutally," "sadistically," "savage," and "barbarically." Other possibilities might include "inhumanely," "heartlessly," "callously," "ruthlessly," or "mercilessly." Depending on the context and tone of the writing, different synonyms might be more or less appropriate. For example, "viciously" might suggest a more active or intentional form of cruelty, while "mercilessly" might suggest a more indifferent or uncaring approach. Ultimately, the choice of synonym will depend on the writer's particular purpose and style.