This guide will tell you how to get the Supreme Helper Minion achievement easier than every in the new Journey Mode introduced in Terraria 1.4.
Guide to Get Supreme Helper Minion Achievement
First off, you should start a Journey Mode world as that will help you speed through the angler quests by duping the quest fish. I started farming the quests post-Moonlord but you could start whenever you like as long as you have the angler NPC. I would recommend doing this in the endgame as some of the steps will result in spawning events and invasions that would significantly hinder the progress of players with less gear. Doing this in the endgame also allows for better tools which can be used to carve out large fishing ponds quicker. As you progress through the game, you'll encounter many fishing poles to choose from. By endgame, I will usually have the Sitting Duck's fishing pole which is obtained through the Traveling Merchant. Break any rocks you find and catch the worms that come out with a bug net which can be obtained from the regular Merchant. You'll want to craft them into Enchanted Nightcrawlers for decent bait. After that, it is quite helpful if you brew fishing and sonar potions as those will help lessen the time.
When you start the first quest, go to the area specified by the Angler where you can find the quest fish. When you reach the correct biome, try to find an existing pond to fish in. Usually the pond will be quite small - around 100 blocks of water. What you'll have to do next is dig out the hole so that it can hold more water. Dupe some buckets of water and then fill it up such that it is around 300 blocks (you can count the buckets used up to figure out how much there is). This is important because as a lake with 300+ blocks of water will give no penalty on your fishing power. Now it's time to begin fishing. Once you catch your first quest fish of that type, research it. This will result in you only having 1/2 of the quest fish researched. This is actually a good thing because now the fish is out of your inventory meaning you can catch a new one. It is extremely important that you do this because you won't be able to catch another quest fish until it's out of your inventory. When you catch the second quest fish, you can fully research it and dupe it to be given to the Angler.
In the last section, I talked about how you could dupe the quest fish to be reused in later quests. This is one of the key reasons why Journey Mode is important for farming these quests. The other reason is how you can skip to the next day with the time feature in Journey Mode and effectively change the Angler quest. Once you have fished up and duped your first quest fish, it's time to move on to the next one. Press 'C' to open up the Journey Powers menu and click on the time settings. Click on 'Midnight' and then click on 'Dawn' this will cycle to the next day and you can start on the next quest immediately. Repeat the same steps and keep duping more fish until you have a large collection of them in your duplication collection.
Later Fishing Strategies
Once you have accumulated a large collection of duped quest fish, you don't even have to fish anymore. You can just stay by the Angler and quickly cycle the days while you hand duped fish to him. To speed up the process even more, input "quest" in the dupe search bar and you'll be given all the quest fish to choose from. Use 'C' to open the Journey Menu after you've handed in a quest fish and use you mouse to change the time of day. Use shift to quickly put the correct fish in your inventory. After you do this for some time, you'll get the hang of it. This entire achievement which should have taken many days will now take a couple of hours.