ReCore

I found a really easy way to get this achievement in the Warren dungeon which you have to go in on the way to E Tower in the main story. You will be collecting 4 parts to build a flyer bot ,after gaining the second part from a chest it will show a door being opened to your right. When you go through this door you will see a bunch of small flying red enemies making a barrier so you can't go through to the green objective marker. Basically there are red enemies flying above this so as soon as you shoot a enemy in the barrier with red lasers they will be replaced straight away by one of the enemies flying above the door, so all I did was just keep shooting the bottom right two and they would get replaced with new enemies flying above. Keep shooting until you hit 40x combo. I have just tested and can confirm this works after you have cleared this dungeon in the story!

Check out the video below thanks to Maka!

There is a fantastic place to get both Juggler & Master Juggler while playing through the story. During the mission in THE WARREN you will come across a door protected by flying enemies that will constantly respawn (as long as you don't destroy their spawners above the doorway).
Simply shoot the enemies in the door way until you have a combo of 40 and both achievements should unlock.

This can be obtained on the very first 'boss' enemy of the game, where you're trying to take it's prismatic core. If you're already past this, just start up a fresh game in one of the other two save slots. At a certain point (25% boss health), you will hear "It's healing itself. The only way to beat it is by pulling out its core." This is where you can go into a nearly limitless combo by simply shooting the boss until it reaches it's 'health break' status and has its core exposed for extraction. Not to be confused with 'shield break', which does not increase your combo count. Health break is when it hits 25% health again and its core is exposed.

When this happens, hold to lock onto it and to fire until its health reaches the two triangles and 'breaks'. This gives you one combo counter. Immediately click to initiate the tugging sequence, and just hold down on right stick so that your tether breaks as fast as possible. As soon as your tether breaks, hold and begin shooting again as quickly as you can. The time for your combo to expire gets shorter and shorter as your combo gets higher, but it is doable here.

I've uploaded a video to youtube of my first (failed) attempt at this. My combo fails at 25, then 33. I accidentally didn't record my next attempt at it, where I actually reached the 40 count, but this video will show you exactly how to do it. You will also get the 'Juggler' achievement in the process if you didn't already get it.

Further to the other posted solutions, if you missed it in The Warren during the story, it doesn't become available again until after you've finished the story, so whilst still grinding away at the story, in the Eden Tower, Floor 2 there's another opportunity. As you enter the first battle arena part, in three corners are these fly-spitting pods. You can easily get 40+ combo by shooting the flies, not the pod (they're one-shot kills). If you fluff it up on one of the corners, go to another one. Be warned, at the start you SHOULD have enough time before big enemies spawn, but if you don't, leave just one alive and head over to the corner again and have another go. Plenty of chances. IF you manage to shoot all the pods by accident, die and retry, minimal reload time and you're back with three more attempts.

https://account.xbox.com/en-us/screenshot/dffb645c-4d05-46c1...

Another easy way to get this, is in the House Of Steel dungeon. One of the corridors is blocked with those "gridbots". Hit the first grid with a charged shot, immediately followed with a charged shot to the second grid. This way I achieved a 46x combo.

Just to add to the above guides a cheesy way to get this is to find the ORANGE IRON TUSK SP-DR set for Seth. Once you research and equip this will boost your combo duration by a massive 40%. I obtained this achievement 10 minutes after equipping this on the second floor of eden tower

Another way that I find both reliable and easy to obtain the achievement is the brood hive. Once you unlock brood hive and walk through the first door past violet you will be met with about 20-22 (1 hit bots which you can get an instant 20-22 combo). There is a pillar in the center of the room in which about half of the bots will run one way and the other half will run the other way. I recommend to NOT kill all the bots before you are standing at the door for the next room as the combo timer will drain quickly and you may not have enough time before the timer drains. Just stand under the doorway and as soon as the last bot is shot drop down the edge and start shooting basically anything that moves as more 1 hit bots will spawn on the ceiling as well other Corebots that can help keep the combo going.