The Ouroboros-medal was a one-time medal earned by Agents who were at least Level 3 during Year Six of Ingress.
Mechanics[]
This medal was awarded to all Agents who are Level 3 and beyond on the 26. November 2018. This counted by an Agent's level at the time. All actions counted, so long as the Agent was at a specific level, they will earn a tier. Bronze was set at Level 3, Silver at Level 9, and so on. Onyx/Black was earned by all 
recursed Agents, despite their level.
Strategy[]
This medal is no longer earnable, since it has been discontinued.
Typically, there is a double AP event before these medals are released. Since this medal counts an Agent's level, it is advised go for anything (legally and fairly!) that will make them level up quickly. Advanced fielding, destroying enemy Control Fields Δ, capturing Portals will do. A plan will make it infinitely better and more efficient. If an Agent is at least Level 8, an Agent can also aim for medals they are missing. By achieving higher-tier medals early, it removes the obstacle of needing to grind a specific medal. Otherwise, it can potentially put them a tier down on this medal.