To unlock this achievement, you have to beat the first 2 phases of the Resourceful Rat boss fight.
The first part of the achievement involves actually getting to the boss. Your first goal should be getting the 6 Infuriating Notes
in order to navigate the new bonus floor where the Rat is the boss. They are found randomly in chests, except for the 6th, which can only be dropped by mimics once you have the first 5. You only need to find each note once; they'll be in the Ammonomicon to look up later. On each note, there is a piece of cheese pointing in a direction, and they are unique to each player.
You'll also need to unlock the Gnawed Key, which you can work towards while farming the notes. The Gnawed Key is available for purchase in the shop on the Second Level - the Gungeon Proper. It initially costs 1000 coins, but you don't have to pay it all at once. Just keep putting money towards it when you can. Once you unlock it, it will always be available for 115 coins in this store.Note: You can also steal the key.
If you steal the key before the finish the money grind, it will return next run at whatever price it was before you stole it. Given its price, it can still be useful to steal it even once you've finished the grind.
Now to get to the Resourceful Rat's Lair. On the third floor of the Gungeon - the Black Powder Mine - there will be a room with a few rats running around. In this room, there will be a trap door that starts the Route to the Resourceful Rat's Lair. You need to either use a blank to reveal it, or run around the room until you notice the dust cloud you kick up is bigger than normal. Just keep running around this area until the door is revealed. You'll need a regular key to open the trapdoor.
Once you've opened the trapdoor, you'll be in a long room with just a minecart track. When you ride the track to the end, you'll need to break the secret door to the North. To do this you need either:
- A Blank
- Armor (jump into the pit and it triggers a blank-like effect)
- Lil' Bomber or the Bomb Active Item
Once you've broken this wall, you'll be in a short hallway. There is another wall at the end of the hall you must break. Jumping into the pit won't work here, but if you have a way to injure yourself near the wall, you can still use the armor trick. Once through this wall, there's a trapdoor to open with the Gnawed Key.
You are now in the Resourceful Rat's Lair. Go to the next room and there will be a pit to jump into. Once you've jumped into the pit, you'll have to check the Infuriating Notes in the Ammonomicon to see which direction you need to go. Follow the directions the cheese points to. For example, my notes were - Down, Right, Left, Down, Left, Right. Once you've followed your directions, you'll reach a room with a few pickups, and a door to the North will lead to the boss battle.
The Resourceful Rat is a 3-phase boss battle, but you only need to finish the first 2 phases. From here, it's just up to you. Here's a guide
to all his attacks. Here's also a sample of me fighting him. I lose on Phase 3, but as I stated, you don't need to win here.
In order to have the best chance of beating him, I'd recommend you completely clear out every floor before his lair to make sure you have a good stash of items. This also helps you save money for the Key (if you don't steal it). This would include:
- The First Floor - Keep of the Lead Lord
- The First Bonus Floor - the Oubliette (Instructions on how to reach.
Make sure you pick up the Old Crest to reach the Abbey of the True Gun
- The Second Floor - Gungeon Proper
- The Second Bonus Floor - Abbey of the True Gun (Must carry the Old Crest to an Alter on the Gungeon Proper to reach. Any damage will cause you to drop the Old Crest)
- The Third Floor - Black Powder Mine