Interesting challenge. Like so many others,,, I like Licorice as a synergist. It's used in TCM to ease the flow of Chi through our system. This can be seen also in Western medicine where it supports the Adrenal glands, helping them energise our body when it needs a boost. Adrenal glands also produce Cortisone and related steroids which are responsible for reducing or masking inflammation. As inflammation is often the root cause of pain and blockage, relieving it helps the blood flow, energising the body, in a sense, easing the flow of Chi and blood.
While Lavender isn't traditionally listed as a synergist, I use it in so many of my remedies in a supporting role that it may as well be one.