Credit to Phenix Nocturne for posting his strategy for this achievement in the comments of the previous solution. It seemed worthwhile to pull out that info and create a standalone solution.
Step 1: reach level 60 with the Monk
There is a bug(?) where you cannot unlock this achievement by capturing the only lvl 40 monster in the game. Instead, you must capture The Living Creature, which is lvl 60. Level 60 requires just under 99K total experience, so prepare to grind for XP.
A few tips to help with leveling up:
- Find a Lexicon ring, the earlier in a run, the better. This ring provides a 30% boost to experience gained and every bit helps.
- Kill those shopkeepers! These guys are worth 300xp and can be found in a level more often than not. He can show up at white alters and inside hidden rooms.
- Kill the Warlock! He's worth a whopping 2000xp.
- Talk to guitar-playing skeleton. For 100g, you get a random amount of XP. I've gotten anywhere from 1 to 300xp.
Step 2: assign the Mana attribute to all three upgrade slots of the Monk's fireball special move.
This will enable you to completely fill your mana bar and execute the soul capture attack on The Living Creature.
Step 3: die (or finish the game by defeating the Warlock).
This will put you in the black and white area, where you will then choose to enter the third area (Sacramentinos Cathedral). The Living Creature has a chance to appear each time a new abyss is generated and you enter this area. If he appears, proceed to Step 4, otherwise, repeat Step 3.
Step 4: capture The Living Creature
This technique is quite simple, and very cheap. As soon as you enter the cathedral, quickly jump up the scaffolding at the start of the area. Once at the top, proceed right until you are in line with the first candelabra in the background. Now, facing to the left, start unleashing the Monk's fireball special attack. The fireballs should bounce down to The Living Creature, who will try dodging wildly. Adjust your position as necessary to make sure the fireballs are reaching the lower level and have a chance of hitting The Living Creature.
Keep throwing fireballs until your mana is full. Now it's time to jump down, unleash the soul capture special move (X + Y) on The Living Creature, and then finish him off through attacks of your choosing.
Pick up his soul and the achievement is yours!