I've tried taking down Dreadnaughts with each class as well, and the Melee seems to be the one with the biggest problems since you have to get in up close and personal to deal damage.

With the gunner it was pretty straight forward kiting, but I wasn't able to shoot off any limbs either (maybe not fully implemented?)

The Heavy is the best one - if you don't use the heavy flamer at least :) Only have the starting Heavy Plasma, but man does that thing wreck him good! I've had some interresting fights and they are scary - but if you manage to take down the adds then he isn't that much of a problem (until you get overwhelmed and knocked back all the time).

Only died once, but that was due to my internet shutting down :D

I find that you need to clear the room.   As melee I normally jump about killing the ranged mobs before engaging the dread.   A thunderhammer works well, you need to deal with the knockbacks/knockdowns and taking a boatload of bolter shells to the face.   Currently I am running round with a poweraxe and shield as well as a heavy flamer.

Skip to about 4mins 50 secs on this vid to see me fight a dread.   Running with a multi melta/power axe and shield in this recording.

https://www.youtube.com/watch?v=dXM2VXHIpvU

The decimator fight is more interesting though as it has shield pylons aswell as additional npc mobs to deal with, however i found again killing the add mobs first, then literally hiding behind the pylons so the decimator kills them for you made it pretty straightforward.

depending on the build I have found if playing ranged, dropping a tarantula at the entrance to set up a trap the most effective mean to distract it and pummel it while it is attacking the tarantula rinse and repeat backing away a room at a time usually by the 2nd tarantulas demise the dreadie is almost done.

running cyclone launcher armour I dont back off as much and run a decent armor eating heavy like plasma cannon or lasgun.

melee I found most fun and just went toe to toe shield and power sword with jump pack when I was supressed jumped out and retreated to restore supression then went back in after a nade.

Have to agree with the OP, got owned a few times and burnt through health pots. It's much easier to clear the room and then go toe to toe with the dreadnought.

BUT its really not a fun fight (i'm playing melee) and basically i spam attacks, get knocked down, get back up, spam more attacks... repeat until he dies :(

Maybe something could be done a la The force Unleashed and/or arkham asylum (and possibly shadow of mordor though i'm sketchy on that one) whereby you have to hit a certain button combo after taking off x health to perform a more effective combo.

Dont know about range weapons, but, if books are not lieing, melta must be a good choise for this. 

How this would translate into the game, would be that at full health a chaos dread would go all out without regards of own troops, at 75% it would go to a melee stance, use own available troops as backup. At 50% health it would back out and start using ranged attacks, allowing own melee forces to try and keep the player at bay, at 25% it would start to think about a tactical retreat again using own forces to back him up.

shtompy smashy

https://www.youtube.com/watch?v=32XcM6BRvpc

Have you guys seen any other dreaddies from different factions yet ?

Without any significant CC, such as slows / snares / stuns / knockbacks / interrupts, and without enough DPS to simply saw through it, it's simply a case of kite, rinse out all skills, repeat. 

I would consider revisiting this once some of the itemization issues settle or are re-examined - having more Attack / Defence might mean it is overall easier to take down the things. I've also found switching to a 2h weapon helps taking them down quicker, albeit with the effectively the same method.