Here is the super easy to follow guide to find the one and only Legendary Dopefish in AMID EVIL!
He Swims, He Hungers Achievement Walkthrough
Finding the Dopefish is quite easy! The only requirement needed to be done before you can find him is you need to complete Episode 1 and 2 to unlock the next area to the hub. I will also state that difficulty does not matter!
As you pick your difficulty and enter the hub area, move into the next hub area. The stone pillars will lower when you are able to enter this area.
From the water pool that has the text on the pillar, run to the right and to the corner to where the "Portal To Arcane Expanse" is located. Run up and look behind it!
In the left side (if you were to look from the front of the portal) there is a little hidden path in the water. Swim in there! You will find a secret portal to a secret level!
Now this first half is fairly easy. The level is very linear and doesn't have anything hiding (that I've seen) so it's pretty straight forward. You will be stripped of your weapons and armed with only your axe. You will start out swimming. Swim up to the area ahead, to the right will be a opening with a red light, and some mines. Swim in there while avoiding the mines!! You can't afford to lose ANY health!! More will be explained, quicksave if you have to, but this part is fairly easy to avoid. Once you get through you will get out of the water and have to climb up a wall. Easy, just don't jump in the lava.
You will then come up to some fireball shooter traps. Easy to avoid, just wait for them to shoot, and move one by one. Again avoid being hit, you can't afford to lose health! Once past the fireballs, you have to jump down and run into this canyon area with a cylinder shaped building, run to the right side as there is a path that leads up to a ledge that will let you jump into it.
Now this is the biggest pain in the ♥♥♥. QUICKSAVE Here! If you fall you won't die, it will just teleport you back to the platform you are on. You have to avoid getting hit by the crushers and jump through them. Try to time it as best as you can. If you get hit you will probably get shoved off and had to retry, but if you take too much damage, it is better to just QuickLoad.
Once you get past the 4 crushers, there is a platform there where you can stand and be safe. Save here again! The next part is 2 pillars that rotate clockwise across the platform. Do not touch them! They will push you out of the way (and deal damage) and make you fall, which will teleport you back to the start of this part. This is why I said save before this part. Try to time it, and jump past them, and enter the portal just behind them!
You will now be teleported to a new area. Don't worry about falling, you will just be teleported back here with no damage. We are almost there! Follow the path up till you see this opening. The path you need to go is in the water, with more swimming.
*Extra bit* There is a portal to the right of this opening (you can see a purple glow on the rocks) this portal just takes you back to the canyon area. I don't really see a point to it.
Swim through the water, and once you get out, you will see the prize waiting for you! The Legendary Dopefish! The achievement is yours!! However!
Ah Ha! It't a TRAP! You will get a nice surprise of a familiar face...es! Come on! Do it! Do it Now!! *Dabs*
Once you have dispatched everything. The platform that held the Dopefish will lower, drop down there to find the exit. And BOOM you completed the secret level!
You can EASILY get the "Hardcore" achievement here as well!