Recent experimental studies have shown that radio links between low-power devices are extremely unreliable. In particular, the instability and unpredictability of low-power radio links in dense wireless sensor networks(WSNs) makes it extremely challenging to improve energy efficiency for reliable communication. In this paper, we propose a novel energy efficient cross-layer based scheme for WSNs. We consider the joint design of the physical(PHY)layer and the medium access control (MAC) to maximize network lifetime under reliability constraints. Our approach consists of opportunistic transmission power control and cell-based efficient time slot assignment. Our approach can support more reliable and energy-efficient data transmission in dense cluster-based WSNs. Performance evaluation results show that our cross-layer based approach outperforms traditional layered protocols. Furthermore, our approach is more energy efficient and can maximize the network lifetime. The network lifetime in our approach has more than doubled compared with the current scheme.