The title sums everything up, but if you want the long version:

I can provide hundreds of screenshots that prove a few things.

1: I, like many other players, can consistently solo all 3 stages of a T3 arena, and in all 3 stages of it, will both enter and leave with 100% full health, without even using a single pill.
1#2: But only if it has no devilites or greavers, the fastest enemies in the game.

2: Devilites are the deadliest common enemies in the game, now that greavers no longer spawn in groups.

3: The fiends group of enemies is the most deadly group in the game by a huge margin.

The problem:
Using the 7-2, 7-3 and 8-1 missions as examples, (The 6 that focus on the 6 individual monster groups), I entered the 4 7-2 and 7-3 ones with full health, and I left with full health and 3 unused pills in my bar. On my first try on the 8-1 T3 fiend mission, about 5-10 minutes in, I was overwhelmed by roughly 8-10 devilites and 4-6 spookats in a confined area. I entered that area with almost full health, (Took a hit from a group of devilites before; aside from greavers, they're the only enemies that can consistently hit me and other players), 3 pills and both tetra pendants on. 1, maybe 2 minutes of dodging, weaving, shielding and barely managing to attack back a single time, I used up all 3 pills, and I was still back to below 50% health. I had to return to haven rather than pointlessly die and waste energy. This is where the problem comes into play: when you're surrounded on all sides in close quarters by slimes or gremlins or whatever-enemy-that's-not-a-fiend, it's no problem. You can focus on all 10-15 of them, or even all 25+ of them if there's groups of mini enemies, like scarabs, because they all have relatively slow movement speed, or relatively slow attack speeds, or both.

Devilites (and greavers) have neither of those things - slow movement speed nor slow attack speed, so you can't focus on all 10-15 of them simultaneously. They deal all the same damage as common enemies, they spawn in the same groups of 1 all the way up to more than 15 simultaneously, depending on the mission, and yet their movement speed is insanely high, as is their attack speed. When you have 8, 12 or even 16+ enemies all attacking at different rates on all sides, it's not so bad, as long as they're slimes or robots or what have you. If they're greavers or devilites, you will be hit. Repeatedly. Maybe even to the point of dying. All because,

1: Their attack speed is so high compared to other common enemies.
2: Despite this, they do just as much damage as other common enemies with slower attacks.
3: Despite both of the above, they can and almost always will move faster than you do. A perfect example being just a few minutes before I started writing this. I joined a friend in the Whipping and Mishandling mission, a 6-2 one. On the 3rd level of this mission, there was this area: http://i.imgur.com/GhAceDq.jpg Due to extremely bad luck, after 3 levels in a row, I didn't come across a single heart capsule to boost my health, and out of 2 pills found, I took 1 and my partner took the other. As you can see, I've already taken 1 hit, leaving me at 74% health despite 4* shadow armor, 4* shadow helmet, 4* shadow shield and 2 tetra heart pendants. In that picture, I fire 1 shot. That's one of the only shots I had time to fire out of all the 30+ seconds the fighting lasted.
4: And despite all of that... Three Rings still wants them spawning in amounts ranging from 1 to as many as 20 all spawning at once.

At one point in the fight in the above screenshot, I was here: http://i.imgur.com/RwFFFrD.jpg (I'm invisible due to flickering while taking damage. This was the moment my shield broke.) In this picture, I have a fiend circled in blue with two arrows nearby. This is the path the fiend took when strafing. You know, that extremely high speed movement, almost teleporting, from one spot to another. It did this twice. First, from where it's circled to where the 1st arrow leads, then from where the 1st arrow leads to where the 2nd arrow leads...

And in this picture: http://i.imgur.com/bCpppK8.jpg It did it 2 more times in a row. It started behind me and to my left. On the 3rd 'teleport' let's call it, it actually ended up in front of me, because hey, why not give groups of enemies that spawn in packs of 10+ the ability to teleport faster than the player can move. On the 4th teleport, it finally paused, allowing me to pull back ahead of it and fire off that 1 shot, though its friends were still happy to keep spamming their ranged attacks at me all the while.

To give you some insight into what this was like, check out this picture, taken 6 and 12 seconds before those other 2: http://i.imgur.com/iGr1Yav.jpg On-screen, you can see 7 devilites targeting me to attack. You can even see weapons being thrown toward me from off-screen by 2 more devilites on radar/the minimap in the topright corner, for a total of 9v1. Give me 9v1 slimes. Give me 9v1 gremlins. Give me 9v1 robots. I swear, 9/10 times, I'll come out either with the same health I had when I came in, or one of 'em will drop a heart capsule, I'll pick it up, pick up some hearts they drop and walk out with even more health than I had when I came in. 9v1 devilites? Nah. For all my dodging, for all my kiting, for all my shielding, for all my gear being 100%, well, 'geared' toward defending from these exact enemies, those screenshots show it was no good. 9+ of them actively fighting me. I didn't manage to kill a single one before they got me to slightly below 50% health. I was going to die, so once again, for the 2nd time this year and the 2nd time this week, I had to return to haven before I died a pointless death and wasted energy for no reason. Looking back on it, I have no idea how I soloed that mission without dying. Maybe it has a split path, I got lucky and ended up on a level that wasn't that one; who knows. Whatever the case is, this time around, with a dead partner not contributing anything, I had no such luck. 9 devilites, not counting the 4-8 off-screen who weren't actively trying to kill me, vs. 1 human player = 1 dead player, given enough time.

When you experience firsthand how these enemies bring you down to 74, then 48, then 22% health, you use 3 ultra health pills and 1 status cure pill with 2 tetra heart pendants and full shadow armor equipped, then have to leave the mission anyway to avoid dying, it's extremely plain to see that fiends - specifically devilites and perhaps still greavers; I haven't yet had a chance to see what the new greavers are like compared to the old ones - are vastly overpowered compared to any other enemy in the game. Give me a mission with nothing but gremlins and robots, and I'll come out with full health and 3 unused pills in my bar. Give me a mission with nothing but the undead and slimes and I'll do the same. Give me nothing but fiends, and I'll take heavy amounts of damage despite my dodging and blocking, and will not come out with 3 unused pills.

How to solve it:
There are plenty of options.
1: The most obvious one being, "Do what you did with greavers just a couple of days ago; reclassify them as elite enemies so they no longer spawn in groups of 8, 12, or even 16+." Then again, if you did this, there wouldn't be many non-elite fiend enemies left, so let's look at other options.

2: Make them focus on using melee instead of ranged attacks, or remove their ranged attacks altogether. They, like greavers, can consistently move faster than the player can, making this a very small nerf indeed; they'd still be able to 'teleport' right up to the player, as usual, and get in a melee attack. When it's 1 devilite vs. 1 player, this is ridiculously easy to block, though very difficult to dodge. But it's rarely 1 devilite vs. 1 player. It's usually 4-14 devilites vs. 1-4 players, making it hell on Earth and almost impossible to escape unscathed.

Since only removing their ranged attack would barely dent their DPS at all, due to their ability to 'teleport' next to the player and quickly use a melee attack, this wouldn't be enough to nerf them down to the same level of difficulty as other enemies in the game. So let's continue looking at more options.

3: Lower the frequency with which they attack. If you're going to have them spawn in groups of 10+ with all of them attacking us from literally every side at once, at least make it so that... Well, let me put it this way. Think of 5 seconds in a fight vs. the usual 10+ devilites. In the 1st second, you're blocking 1-3 of them attacking you. In the 2nd second, you're blocking a different 1-3 of them attacking you. In the 3rd second, you're still struggling to shield and dodge simultaneously while a different 1-3 of them attack you... And your shield breaks, and now you're damaged without ever laying a hit on them. By lowering how often they attack by a large amount, players would actually have a chance to shield against one attack and run away before other devilites attack again, instead of the usual, "Awesome, my shield is receiving 2-4 blows per second and there's nothing I can do about it."

4: Increase the amount of time they take to 'broadcast' their attack. You know how a gun puppy opens its mouth, pauses for a second, then attacks? That gives you time to dodge or shield or both. I'm almost certain that, out of every single enemy in the game, devilites' primary ranged attacks have the shortest pause/'broadcast' time in the entire game. One moment they're standing still, the next, they're attacking you almost instantly. I don't know about you, but this makes it so that, whenever I block their attacks, it's almost always at the very last fraction of a second, and if I had been just a tiny bit slower, I would've been hit. Funnily enough, I don't have this problem with any other enemies.

5: Lower their health. The faster they die, the faster we can fight back against the 10+ of them that are still alive. For example, when 10 are alive, I can barely fire off 1 AP bullet every 5 seconds or so, due to constantly being bombarded by attacks at least once per second, if not far more frequently than that. But when only 2 are alive, I feel safe firing off 2 AP bullets, shielding, waiting for them to attack and repeating. Hey, speaking of killing them quickly...

6: Give us back the old AP. It would finally take 'em down a beg, so it's not, in terms of deadliness, devilites and greavers > every other enemy in the game. Why did you do this in the first place? Let's take a look at this.

2 years ago,
T3 100% elemental guns that don't force you to freeze for 1-3 seconds while firing, unable to block or even move: 5 (That don't force you to hold still for 1-3 seconds while firing, unable to block or even move)
T3 100% piercing guns that don't force you to freeze for 1-3 seconds while firing, unable to block or even move: 1. Argent Peacemaker. It was literally the only one in the game that didn't do that.

Now,
T3 100% elemental guns: 6 (Same, don't force you to be defenseless and motionless for 1-3 seconds)
T3 100% piercing guns: 0 (Same)

All because AP was pointlessly changed from piercing to elemental. Why, Three Rings? 5 good elemental guns wasn't enough for you, and 1 good piercing gun was too many..? It was one of the only weapons that worked well against fiends. Now there's nothing that works well. Now everything is, either it deals low DPS to 'em but lowers the risk of you receiving damage, (I.E. "safe guns," guns that aren't autoguns or a magnus line gun), or it deals high DPS to 'em, (Assuming they don't dodge, which they do 33-50% of the time), but vastly increases your damage received by 100-500%, due to forcibly being frozen for 1-3 seconds while firing, unable to move or even shield. So why not make AP the way it was before? It's objectively a good decision. I'll take 1 good piercing gun and 5 good ele guns over 0 good piercing guns and 6 good ele guns every day of the week, and twice on Sunday.

7: Or just add a new line of piercing guns to the game that aren't "unsafe" ones, like literally 100% of all piercing guns in the game right now. The autoguns and magnus line ones that force you to hold perfectly still and unable to shield for 1-3 seconds during every attack, 100-300 seconds per every level, with no choice but to take damage from enemies. So why not add just one piercing gun that isn't an "unsafe" one? I don't think one is too much ask for.

8: Here's another thing you can do: slow down the speed their projectiles travel at. From what I saw while playing with that friend in the screenshots above, every single attack that hit him was a ranged one, not a melee one. Not only is the warning that they're about to come at you extremely brief, the projectiles themselves move extremely quickly compared to more common projectiles, like the bullets that standard gun puppies and spookats fire. Why are every single aspect of devilites either equal to or faster/more powerful than other common enemies..?

9: One more option you can utilize: simply make them deal less damage. If you're a three rings dev who reads all this and actually decides to nerf fiends, if you only use this option, sure, we'll still be taking hits 500 to 5000% more often than we do from all other enemies in the game, but I guess at least it won't hurt as much... Meh.

The best option would probably be some combination of two or more of the above, if you ask me.

To help back up my words about almost never taking hits from non devilite and greaver enemies, here, have an album of over 100 images of the last T3 arena I ran. Due to having to upload the pictures 15 at a time, about 1 in every 15 or so are going to be out of order, but aside from that, enjoy: http://imgur.com/a/4WNeN#0 Zero hits taken throughout all the fights, so zero pills used. Just before stepping on the party button in the first room, I set down a spike shower bomb. After that, I only used 2 guns for all 3 sections of the arena.

Don't hesitate to post below and let me know what you think about all this.