Undervolting is the only way to significantly lower the power consumption.
Decrease clocks and/or increase voltage. That's the usual stuff to make GPUs stable.

I would definitely not try this if you don't have any way to remotely control the rig (IPMI or a watchdog). Most AMD rigs mine memory-intensive algos like ETH or Cryptonight which don't put a lot of stress on the GPU. So when you run something compute-intensive like Lyra2rev2, there's a good chance that your OC won't be stable and you'll have to do some adjustments (core clock/voltage). You could easily get your rig frozen when you run this miner for the first time.