Multi-agent reinforcement learning (MARL) faces two critical bottlenecks distinct from single-agent RL: Credit assignment in cooperative tasks and partial observability of environmental states. We propose LERO, a framework integrating large language models (LLMs) with evolutionary optimization to address these MARL-specific challenges. The solution centers on two LLM-generated components: A hybrid reward function that dynamically allocates individual credit through reward decomposition, and an observation enhancement function that augments partial observations with inferred environmental context. An evolutionary algorithm optimizes these components through iterative MARL training cycles, where top-performing candidates guide subsequent LLM generations. Evaluations in Multi-Agent Particle Environments (MPE) demonstrate LERO’s superiority over baseline methods, with improved task performance and training efficiency.