This seems to work about 50% of the time.
Starting from Nalbina Town/West Barbican (it's the nearest place with a teleport crystal), head south to Dalmasca Estersand/Yardang Labyrinth (the location of Ripe Rampager). Head towards Dalmasca Estersand/Sand-swept Naze using the shortest route; you should come across a cactoid, which appears from being buried. Kill it, and then head towards Dalmasca Estersand/Outpost, but don't go through the transition just yet. From there, you will find a cactoid to the south. Head straight towards that cactoid. If the Ripe Rampager doesn't appear now, you will need to kill another cactoid, or maybe two. Kill this one, then head towards the area with cockatrices in order to find another buried cactoid and then kill that one. Head back to the cactoid area south of the Outpost.
Feel free to kill any annoying wolves on your way - they don't seem to have any bearing on wether the Ripe Rampager appears or not (I have always killed them myself). If your last kill is a cactoid, that doesn't seem to affect its appearance, either. This rare game can be met quite early on in the game. I have been unable to determine if casting Stone on the cactoids has the same effect as killing them. Stone would be a better way to kill the cactoids as you would be able to kill as many as it takes to get the Ripe Rampager to appear. Note that the rare game is a different monster category to the cactoids, so your chain will break when you kill it.