A Revenge Medal is awarded when you kill someone who has kill you three times in a row. Start an Internet FFA Deathmatch game on "Railyard", which is a relatively small map with Railgun spawns. Add a bot then enable the following codes.

bot_gauntletonly 1 (forces the bot to use the Gauntlet)
g_damage_g 500 (Gauntlet does 500 damage)
g_damage_rg 500 (Railgun does 500 damage)
fraglimit 500 (500 fraglimit)
timelimit 120 (2 hour time limit)
g_speed 600 (maximum run speed to 600)

Allow the bot to kill you three times, then Railgun it. Repeat the process as many times as needed.