Today I was "Unbeatable" in PvP for the first time, accompanied by the normal "An ally is unbeatable!" announcement. Upon Googling it I found that this is awarded when you get 4 medals in a single spawn (without dying). However, I had 7-8 medals at the time and hadn't died all match, so the information I found elsewhere would appear to be wrong.
Does anyone know exactly what causes you to become Unbeatable?