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Perhaps I might have some bad luck, but it seems like Venerable Dreadnoughts and flying Blood Angel dreadnoughts are an achilles heel. Bjorn the Fell Handed is especially hard to kill. Even though it is easy to score hits on them with lots of dark lances, a group of dreadnoughts can give each other cover and multiply their survivability greatly, while also providing a wall of cover for infantry as well.
What are some effective methods to deal with a group of dreadnoughts?
"Courage is not the absence of fear, it is the conquest of it." -Anon
squad of wytches to use as a tarpit with haywire grenades. venerables are harder to fell granted but when their not killing any of your guys and your doing some damage its a win. haywire blasters and especially heal lances do extremely well.
1st, vehicles only get cover if more the 50% of the model is out of line of sight (even if its in area terrain.) So depending on terrain you shouldn't have much trouble moving in such a way that he doesn't get cover
2nd move on both sides, or fly over the walker to get side or rear armor.
3rd you're faster then walkers with most of your army. They drop one in a pod: you turbo everything away. Now they have this expensive walker doing crap because it can't keep up.
4th the BA dreads that move like jump infantry are furiouso dreads and they need to activate a psychic power to do it. They're nasty but I don't think they're venerable. Take these out early.
5th dreads on flyers: kill the transport. It's not venerable. Now the walker has to ... (wait for it)... walk.
You illustrate a good point DE don't have much against walkers in CC. Basically it's HWG or nothing for most units. Here's a roundup of what we can do:
If you run a gunboat squad then shoot with ravager and 4 blaster trueborn squad should be able to shoot em dead
If you run a WWP squad- Use the talos. 1 vs 1 the talos wins every time vs a dreadnaught in CC. S7 +2D6 armor penetration for D6 attacks (i recommend chain flails)
Wyches- Wyches with HWG and Shardnets (givem -1 attack and autoglance)
Archons- yet another reason why all archons need HWG
Beasts- Razorwing Flock with Rending
Void lances are surprisingly good at dealing with walkers, namely because the ability to penetrate on a 4+ is extremely useful.
Other options include heat lances (alas they are best used on reavers in this context - scourges using them to maximum effect will be within charge range) and haywire blasters due to their ability to deny a dreadnaught's ability to fire nearly all the time.
The talos is also an excellent walker killer - only the librarian dreadnaughts really pose a threat. A talos with chainflails and either a heat lance or haywire blaster is dirt cheap and will more then likely chomp a dreadnaught in close combat. If it can get furious charge and the charge (not a reliable thing I would say) it really will tear dreads limb from limb.
Lastly (and has been touched on) dreadnaughts providing cover to each other should only work on one particular facing (typically front), so finding one that isn't obscured or an angle of attack in which their formation does not provide cover should be easy. Difficulties arise when they for a static gunline in area terrain, that is a world of pain no matter how you approach it.
Don't knock Haywires, they are dirty, sure a ven dread may make you re-roll the damage, but hell if you get lucky and 10 wyches is likely to get quite lucky that could easily be 4 or 5 re-rolls in a couple of turns, he won't make all of them discount...
Haywire greandes vs dreads are a horrible idea imo.
Against a ven dread you need 6 to hit then 6 to pen then 5/6 to kill then 5/6 to kill again because of the reroll. You could try glancing him to death but still, we're talking fractions of a percent to get that kind of result. Odds are your wyches won't be getting free any time soon. Given wych squads + transports will out-cost walkers by a long shot it's more likely they will be trying to tar-pit you than vice-versa.
In the early game you don't have a lot of options, plugging all your dark lances into them is an option but it's not great.
TBH the haywires seem like the best option, it's just that wyches are a poor delivery vector. Scourges with a pair of haywire blasters can be relied on to score at least a glance. Pretty much any of the results are good for you, stopping him from shooting negates the majority of the threat while immobilising him prevents him from tarpitting your assault troops. Early game just contain their threat.
Late game you have plenty of options because you will be overrunning his positions with assault troops.
The best is the archon soul trap combo to get him up to ST10. He can break what he likes at that point. However do keep in mind the rear armour is just 10. Zip one of the empty raiders round the back and plug him. 2s to glance? yum yum.
With the stormraven/dread combo, you need to immobilise fast transports like that on turn one or whenever they show up from reserve. They are too powerful to leave alone. They are also an expensive enough nugget that you can justify multiple lance/haywire blaster shots to take them out.
Copy, Improve, Innovate
2) 50% of available walkers have an AV less than 12
3) It's only instant death if the walker has a Dreadnought Close Combat Weapon, and not all walkers are equipped with these.