This is a relatively straightforward achievement if you are trying to perfect the game. While searching for all the secrets/chests/map completion, you'll come across almost every enemy. One hiccup that I ran into was missing four or so lighting bug like enemies in the Deep in the Copper Forest area over some spike traps. I can't recall seeing them there the first time so they may spawn after the area (Need confirmation) or maybe I'm just blind. Some minor spoilage ahead:
This achievement does require you to kill Bob which can be a bit of a challenge. If anything, I would say that he was probably the most challenging fight in the entirety of the game. He has 10,000 HP which makes him the tankiest enemy in the game. In order to gain access to him, you need to acquire the Headstone that goes to the statue towards the end of Lake Anna. Acquiring the headstone can be a bit of a pain as well because it's sealed behind a 5 by 5 sliding tile puzzle. I don't particularly have any tips for solving that puzzle because it took me a good 20 minutes before I surprised myself and got everything in the correct spots. Anyway, once acquiring the headstone, head back to the Lake of Anna and put it on the statue. This will open a portal that once you step through, will lead to a square-like room with water in the middle. This will also complete the secrets and map for the area as well. IF you walk up to the water, a prompt will ask you if you want to begin the battle. I suggest saving before doing so and optimizing your group before doing so. The difficulty behind Bob comes from the silencing effect that occurs when he dips below 6,000 HP. The second he goes under, water begins filling the room which gives you a relatively short amount of time before your entire party is silenced. This gives you access to only the three basic attacks and prevents you from casting any spells. Additionally, you take damage every turn because your essentially suffocating. However, with proper planning, this may only be a small hindrance. Before beginning the fight, optimize your party's damage as best you can while sacrificing the other stats. During my fight, Bob literally missed every single attack he threw at me. Next, equip your strongest character with herbal tea and another character with the revival potions. I chose my other two characters to have damage over time potions such as the poison one and flame one. Once my party was silenced, I activated the team spell that causes frost damage every turn and hit Bob with both fire and poison potions. As the fight went on, I kept making sure that the team was in good health with the herbal tea. I also would sacrifice my weaker party members with less health so that i could use the revival potions on them. I found it more useful to use the herbal teas on my tankier members. Overall, managing your party's basic damage and proper potion choice make this fight ten times easier.
Side note: If you can, try to use a character that has a high chance of attacking twice. This helps a lot in the second phase when you can only use the basic attacks. That second phase is like a race against the clock so any extra damage that you can do helps a long way.