Mages of Mystralia (EP)

The first time you find a Boar Rider is in The Rise when you're heading to Sky Temple; you'll want to get the achievement here as this is the easiest place to get it. The Goblins and the Monster Rabbits both count as enemies, it's easy to get him to blow most of them up, he can also ram them to kill them but it's easier to get him to kill them with the bomb, however if you get a Ram kill you don't have to worry it's just total kills so 1 Ram and 4 Bombs is still 5.

In this area though there is 3+ Goblins, and 2+ Monster Rabbits, These enemies do not respawn later so get it now.

Boar Riders aren't Common, they show up in a couple places, later in the game you can find them if you travel to The Highlands (not by fast travel), you'll find a Boar Rider with 2 Goblins, there are also other Goblins you can kite over to the Boar Rider and plenty of Monster Rabbits you can piss off around him to kite to him as well. This one will respawn if you leave and come back as long as you don't fast travel to the area.