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The best, but not only way to get this achievement is by stunning and riding a Razorfire Boar found near the Razor Girl statues, located south of the Wall of Amps and Northwest of the gates to Pleasure Tower.
Get the boar, and drive it all the way to the Sea of Black tears, where you fought Doviculus. There, the most suitable space is driving it into a shallow part of the "Sea" at the entrance to one of the tunnels you used to drive your Deuce in and slay the beast.
You can jump off before you yourself hit the Sea of Black Tears, the boar will disappear, but all you have to do is step into the water, with a single leg. The achievement will unlock once you do so.
TIPS: Popular to belief that boars cannot be slowed, they can. Most people think that left trigger is the only way to brake.
Use B for handbrake, this is mostly used for fast turns but they do the trick of slowing the boar down aswell. This should make it a lot easier getting to the Sea.
Call of the Wild is bound to your technology level, meaning that the more upgrades you bought from the Guardian of Metal, the higher level beasts you will spawn. Razorfire Boars are fairly low level technology so you will not spawn these after you've progressed far into the game.
You can skip the jungle part, so you won't have to worry about Lazer Panthers. After crossing the bridge, take the small ramp after the vista to the right, you will jump over a fence and will be almost adjacent to the transition jungle/undead lands.
It's smart to preserve as much Razorfire Boar health as possible, so do not use any guitar strings that augment your damage.
GUIDE IN SHORT:
Find Razorfire Boar at Razor Girl statues, stun with X, ride with Y. Drive to Sea of Tears, while braking with B. Preserve health, skip jungle. When at Sea, jump off before you hit the sea. Boar will die. Now you step into the sea with one foot yourself, no damage. Achievement unlocked.
Comments or questions for extra clarification are welcome.