Here's how I tamed both of my piggies.
Make sure you have a boat. Find a nice piece of straight river edge and set up an open box of unbuilt fences like this:
~~~~~~~~~~~~~~~~~
~Riverrrrrr~
~~[Boat]~~~~~~~~~|_ _ _ _ <-- Fences, lol.
Grab the attention of a nearby boar and go lure him into the box by standing inside it:
~~~~~~~~~~~~~~~~~
~Riverrrrrr~
~~[Boat]~~~~~~~~~|P _ B _ <-- P is for Playa', dawg. B is for the angry Boar rushing to knock you on your ass.
You'll probably take a hit here, but it's the only hit you'll take until the next round of taming. Now that the boar is inside the box, get in your boat (you left it by the shore, right?) but don't lose aggro. Wait for it to make a failed attempt at moving into the water to get to you (you'll see it twitch) and quickly head to shore and plug up the hole in the box before it tries moving again.
~~~~~~~~~~~~~~~~~
~Riverrrrrr~
~~~~~~~~~~~~~~~~~[P-in-boat]|B _ _ _ | <-- Boar is trapped.
Now comes the easy bit.

Remove one of the
corners of the box. It can't escape, don't worry. On the plus side, you can stand in this space to attack it, stepping back to avoid all of his attacks. He'll never hit you if you time it well.
Now do "Call Thunder" for about 24 Initiative. Step in, hit him with a Sidestep, and step out. Repeat 3 more times. Then, one "Sting Like A Bee" to reset Intensity to 0, and voila, you can use your first Quell.
To re-gain his aggro, get back in your boat and go stand inside the box next to him (get out of the boat). He'll eventually target you. When that happens just jump back into your boat and repeat the taming process, no need to unbuild any fences or do anything else.
The set-up is probably the most difficult part, but once you've got everything ready, the rest is a cinch.

Just be sure you did this on your own landmass, otherwise you'll need a raft to transport your new livestock home.