Punishingly is an adverb that implies something is excessively harsh or difficult. There are several synonyms for punishingly, including gruelingly, relentlessly, arduously, tiresomely, and strenuously. Gruelingly suggests a task or situation that is physically or mentally exhausting. Relentlessly means to proceed with determination or persistence without any break. Arduously indicates a task that requires considerable effort or extreme difficulty. Tiresomely suggests something that is boring or tedious. Strenuously implies something that requires one's full strength and energy to accomplish. Each of these synonyms conveys the same meaning as punishingly, but the subtle nuances between them can add richness and variety to one's writing.