To get this achievement you have to obtain a merciless victory using the Legion with only their three unique perks equipped. You will have to be at least level 10 with the Legion to unlock three perk slots.
Their three unique perks are Discordance, Mad Grit, and Iron Maiden. You will start out with tier 1 versions of these perks, however, I would recommend trying to get these perks to tier 3 to make things easier for you. After level 10 the Entity will start to eat away at the bloodweb so make sure you go for these perks first when they show up.
To obtain a merciless victory you need obtain a rank progress of +2 at the end of the match. The requirements for this change depending on your Killer Rank, with it gradually becoming harder as you go up in ranks:
Rank 20-13: 14-16 emblem points
Rank 12-5: 15-16 emblem points
Rank 4-1: 16 points
Points are awarded depending on the emblem qualities you obtained during the match:
4 points for each iridescent emblem
3 points for each gold emblem
2 points for each silver emblem
1 point for each bronze emblem
0 point for no emblem
This mean that you need at least:
Rank 20-13: 2 gold and 2 iridescent emblems OR 1 silver and 3 iridescent emblems.
Rank 12-5: 1 gold and 3 iridescent emblems
Rank 4-1: 4 iridescent emblems
Gatekeeper emblem: Points for this category are earned by slowing down generator repair progress for as long as possible. You earn points each minute for the first nine minutes of the match, based on the amount of incomplete generators remaining on the map. Ending the trial with incomplete generators also grants bonus points towards the emblem. You will need at least 42,000 for a gold emblem and 55,000 for an iridescent emblem.
1 point per second is earned for having any unfinished generators on the map for the first 9
minutes of the match.
×100 point multiplier for having 5 unfinished generators at the end of the match.
×80 point multiplier for having 4 unfinished generators at the end of the match.
×60 point multiplier for having 3 unfinished generators at the end of the match.
×45 point multiplier for having 2 unfinished generators at the end of the match.
×35 point multiplier for having 1 unfinished generator at the end of the match.
If there are 0 unfinished generators at the end of the match, you do not receive a multiplier
Devout emblem: Points for this category are earned through sacrificing and killing actions. You need at least 6 points for a gold emblem and 8 points for an iridescent emblem.
2 points for having hooked each survivor at least once.
2 points for sacrificing a survivor.
2 points for killing a survivor via mori offerings.
2 points for survivor disconnecting.
0 points for survivor bleeding out.
Malicious emblem: Points for this category are earned by applying pressure on survivors and preventing them from helping each other. You need at least 30 points for a gold emblem and 36 for an iridescent emblem.
1 point for hitting a survivor.
2 points for a survivor reaching a new hook stage.
-1 point for a survivor healing.
-1 point for a survivor escaping your grasp.
Chaser emblem: Points for this category are earned by finding and chasing survivors with short chases earning more points. Also, staying in close proximity to a hooked survivor will reduce your points over time. You need at least 1800 points for a gold emblem and 2550 points for an iridescent emblem.
5 points for finding a survivor.
55 points for hitting a survivor.
90 points for a chase lasting 15 seconds or less.
60 points for a chase lasting 30 seconds or less.
50 points for a chase lasting 45 seconds or less.
40 points for a chase lasting 60 seconds or less.
30 points for a chase lasting more than 60 seconds.
Points drain over time if you remain within a 16m radius of a hooked survivor 10 seconds after hooking them. Staying within an 8m radius drains points significantly faster. Points will not drain when:
Another survivor is within the radius as well.
You are chasing another survivor.
If it’s the last hooked survivor.
If you are on a different level than the hooked survivor (e.g., The Game)
Lastly, equipping items, add-ons, and offerings does not void the achievement.
I hope this helped make it clearer on how to obtain a merciless victory. Unfortunately the criteria has become significantly more difficult since the game originally came out. If you have any questions, do not hesitate to ask. I'll try my best to answer them.