Brütal Legend

For those with lazy eyes or short concentration, scroll down for short version. For maximum awesome and info, read text below this.

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.

First, you need to complete the game (or get near to completion). Next you need to get yourself a Razorfire Boar (found near the Razor Girl statues), stun it and ride it back to where you fought Doviculus.

Doing this is a pain. You don't control the speed of your ride, but you CAN use B to slow it down a bit (for turns, mainly).

When you finally get into the part where you had the final boss fight (through the tunnel, underground, into the big chamber), you need to ride your Boar into the Sea of Black Tears -- without falling in yourself.

The easiest way is to go straight ahead at the ramp at the far end of the chamber. Get a decent run up, and press Y to jump off before you reach the end. Your Boar should continue without you into the Sea.

If it doesn't, don't worry. Provided the Boar goes into the Sea, and you don't die soon afterwards, you will get this achievement. (Even if the Boar wanders in by itself.)

Video help, too:

What beasts are summoned by Call of the Wild depends entirely on how many hunting missions you've completed. Only do the first three, then play through until the Sea of Black Tears.

Next to what will probably be the last bound serpent is a little patch of the sea (the building to the right at 4:37 in the YouTube video above). Go over to the bus, use Call of the Wild, and 4 boars will appear. Hop on one, ride it over to that tunnel, and jump off, letting it continue into the sea. You don't have to touch the water at all; the boar just has to die in the sea after you've ridden it.

When you get Call of the Wild use it every now and then until you can summon the Boars. Do the Hunter missions and after the one where you Hunt the Boars you can summon them.

Then just beat the game. Drive the Duce all the way back to the Sea of Black Tears, then around the Legend there use Call of the Wild, and you get 3-4 Boars. Hop on one and drive it off into the water, jumping off just before so that you survive and it pops after a second. If not you got a few more Boars to work with and can always summon more.


***EDIT***

As Makker points out once you completed the required number of kills for the Boars do not go and turn the mission in to the hunter as you may accidentally fulfill the next level requirement and start summoning those creatures instead.

This is really a sub guide, I also forgot to do it while I had call of the wild, so what I did is drove all the way there on the boar, but I screwed it up and jumped off too soon, the boar drove off and into the sea, but then I got the achievement any way

The point I'm trying to make is the boar has to go into the water, that is all that's important, if you abandon it halfway across the sea of tears room and it happens to drive in, you get the achievement.

Video from youtube