Once you unlock side quests (after the first boss), the person who gives you this quest is sitting on crates as you enter the shopping area of your village, opposite a fountain.
Once you have accepted this side quest, it's time to go hunt the boar in the northern plains!
The boar is located near the entrance to the aerie. He has a lot of health, but this means more opportunity for hits and the good news is you can stun him, so target him, start swinging your sword at him and when he grunts, hit him with a dark lance - this will stun him. Keep swinging your sword until he grunts again and then hit him with another dark lance and repeat until dead.
I worked this out after already attacking him but I ended up with a combo of 160 by the time I beat him on hard difficulty, so if you do this successfully in one combo on normal difficulty or higher you should also unlock this achievement:
Once you have beaten the boar, return to the village to return your quest - the reward is you can now ride boars! Go into the aerie and then back out to the northern plains and you should see a boar and no enemies around. Hop on the boar with
and then just sit still for 5 minutes - no need to go anywhere as if you fall off the boar (from crashing or getting attacked) it will reset the 5 minutes, so don't take the risk.
Video for beating the boar below:
Sorry, this Game Clip has been removed
Happy hunting (both achievements and boars!)