Unsouled

To get to Gnar’s special arena you will need to be hit into the centre part of the wall, the one behind Gnar in his intro (if you are still unsure after that description what I am exactly referring to look for an icon reminiscent of the icon of Lad’s special arena achievement, that’ll be the part of the wall you’ll need to be hit into). If done properly, you will be bashed through that wall and a couple more in a pusedo-cutscene until you reach the special arena (it will initially be foggy and contain poison clouds; as Gnar’s health dwindles these hazards should vanish); defeat Gnar here on Master Soul for the achievement.

Video demonstration (note: you may have to press the third icon on the right from top to bottom, which sends you to a new URL with just the clip, to get it to play):

(P.S After you beat Gnar here, go up to find a statue and hold to get a Passive Soul that is required for Soul Taker.)