Weaken Greavers.

First off, I know that Greavers attacks can be interrupted when they're struck, but let me tell you, that's not always EASY TO DO when they're hording you and attacking at different times AND moving around you not staying in a single position. Oh, and lets not forget that cheap, cheap, cheap dust attack they do, seriously make it so they do it on SOME attacks, not ALL, because there have been separate occasions when I had nearly full health then after fighting a horde of Greavers, remember this: I'm actually pretty good at handling Greavers, I have barely any Hp left, this especially qualifies for RUBY GREAVERS, it's not always fun to be doing well then fighting one horde of Greavers and be near death, oh and then after that LOOK another horde of Greavers! Yay! That's always fun.
Why not increase the radius of the Greavers dust attack and then make it so they CAN'T get through shields? Your shield is your best buddy in tight situations, like being gang banged in Arena's, that being said lets set up a scenario: You're in an Arena in the third match and a horde of Mecha's start gang banging you then a Greaver dust attack (Lets pretend this is a Shock Greaver) gets you through your shield that's hopefully still in tact, you're not going to live, sorry pal.
OH, and one more thing, the range of Greavers is misleading, there range is farther than there attack animation is, please fix that because it's highly unfair.
Remember, I'm not bad at the game, so don't say "Oh if you can't those then you suck" or something else like that.

My method for dealing with Greavers currently is charging my Leviathan Blade then letting them mob me, then I unleash it. That doesn't mean there not overly hard. My biggest issue is the dust attack, it's just cheap most of the time.
Most people don't have a Leviathan Blade at hand.
I don't even have a single sword in my arsenal (except for proto sword and some random beast basher), and getting gang banged by a bunch of greavers is usually bad.
But if I can catch them with a bomb before they attack I am usually good.
Greavers' powerful attacks are balanced by the fact that they have low health compared to other monsters.
Destroy them before they destroy you.
The Spine Cone bomb series has a fast detonation timer, Piercing damage, and knockdown effects for rapid, effective Greaver disposal. The Flourish and Snarble Barb have a surprisingly wide attack hitbox (so it's easier to land a hit on them), and their charged attacks are useful for tearing up clusters of Fiends besides. A Freezing Vaporizer will stop them from moving so you have all the time you need to line up a counterattack. The Electron Vortex will catch them, fling them away, and possibly inflict Shock on them (which can interrupt their attacks easily).
Don't use your shield in their dust. It doesn't help, ever. Just unshield and get ready to hit them from somewhere else.
Depending on which shield you have, you can shove them all away just before they attack. Plate shields have a particularly strong shoving force compared to other shields of equivalent star rating.

The damage and speed is fine, but, like devilmites, the purple thing that pops up is incredibly misleading, and should be tweaked. I generaly can get through a mob with my faust (hooray for being well equiped!), but I have to try to ignore those purple things.
Also you can't just put a disclaimer at the end saying "You're not allowed to say I'm not good at the game" and expect people to not say that. Remember, 90% of drivers think that they are in the top 10%.

I must admit swords help (Barbarous thorn blade being careful to only use 1st slash to avoid moving INTO the mist, or Levi for me), but Shivermist is nice too, and then unleash whatever damage you like (I guess nitro or the DBB work very well too once you've got the timing down).
I'm not much of a gunner, so I don't know what a pure gunner would do...

It's easy to say, charge this sword or that bomb. But that doesn't address the issue. Sure, if you know that they're coming, sometimes you can charge something and be fine. But sometimes they'll hit you before you're done charging it, and interrupt you that way. At that point, charging anything isn't an option anymore. What do you do then?
What I think needs to change is to make the area condition part of the greaver's attack very short term, and blockable. Make it so that if you have your shield up when they attack, you block the condition. Otherwise, leave greavers as they are. They'll still be hard in groups, but at least give players a fighting chance to avoid being chain-stunned or shocked or whatever.

I think they're fine. As you mention, it's all about knowing the layout of a level. And if you get wiped by greavers on a particular level, odds are that leaves a pretty strong impression and the next time you encounter such a setup, you say "aha! greavers might spawn" and proceed with appropriate caution.
I have found that (not surprisingly), heavy swords are a poor choice when attacking fast enemies. Greavers are manageable because they automatically bunch together when swooping, but very unforgiving if you make a mistake in your opening attack. One solution is to use a Flourish type sword; use a 2-swing combo, shield cancel and immediately retreat (or open with a charge attack). The best solution is probably Radiant Sun Shards; it is a fast charging, fast-fuse bomb that when placed correctly, will decimate a pack of greavers. Dark Briar Barrage is also good, although it has a long fuse time, making you very vulnerable while charging the next bomb in a chain.

What I find is particularly effective is pulling a few away from the pack and running backwards, hitting them once if they try to charge at you.

As a gunner, blocking and shield bumping is key. They bounce back when their attacks are blocked, giving you a small opportunity to retreat and fire a shot. If your shield can't take many hits, you have to make use of shield bumping whenever possible to give more time for your shield to recharge.
Greaver do need some tweaking with the look of the dust they leave behind. It can be very difficult to see unlike the spit puddles left by Chromalisks.
Whie they are slow, they have good reach and knockback, but you have to be carefull when you are going to attack as they leave you open for a counter attack. You should time the attacks and counter.
If you use a heavy sword (ex: Sealed sword/Troika lines), it's super easy to swipe at them when they try to mob you. In fact, you WANT them to try to mob you, because that's when DPS with a heavy sword is the best.