This achievement can be completed as a Pyro, on any map and gamemode. But I think ctf_2fort is the best for it.
So, firstly, you enemy team need to have at least one medic, healing his teammates. The medics are kind of rare on public servers, so it's about to have a luck. If enemy team have the medic, healing his teammates by using Medi Gun, just fined them both and ignite. They are more common on the bridge.
The achievement will be unlocked automatically after igniting both of medic and his healing teammate.