What do you do against Double Derp Turtles?

Yikes. Yeah, I am glad to not see that at our store. I am interested in this as well, because if a list can handle this, it should be able to handle almost any double huge base list I think.

The thing with turtles is they will almost always get the alpha, and they will always kill something. It doesn't really matter what you field, they'll kill it. If you don't provide them a decent target, they'll just run around shooting down whatever they want until you do.
You have to force them to engage by presenting a good sacrifice for its melee while pushing scenario, then as above you need a list than can punch back hard every turn. That means buffed banes or buffed heavies.
Turtles are ludicrously overpowered and in best the case scenario you will have to make an even trade for it. Something like an inflictor, a solo and 1-2 dudes.

Zaadesh2 - One turtle gets Inviolable Resolve for +2 ARM. On feat turn, they both have Defensive Strike with a MAT 8 POW 20 auto-KD. Burning Ash clouds could be a problem for living infantry.

Hexeris2 - Banishing Ward goes on one to prevent debuffs. Black Spot is the real threat with them if you brought infantry with poor to average DEF. The turtle charges a heavy (or three), then uses its dual attack to shoot d3+1 shots into the infantry with Black Spot. Each kill is a tail swing on a heavy. If they're really lucky, they can also use the spear attacks on the infantry for another two free tail swings. If all goes perfectly, the turtle can have 10 tail swings in a turn. Again, Cloak of Ash could be a problem for living infantry.

Jalaam - With the volume of attacks the turtles can put out, they can trigger Warpath for his entire battlegroup. DEF 10 may not seem like much, but, when you're rolling one less die from his feat, attacks that shouldn't normally miss them will miss them.

Other warlocks that potential do stuff for double turtles:

Hexeris1 - Parasite lets them really shot heavies off the board.

Makeda1 - Quicken makes one SPD 10, Carnage puts them to MAT 8, and the feat gives them Retaliatory Strike.

Makeda3 - Hand of Death give one Grevious Wounds (on the gun, too) and Overtake.

Mordikaar - Feat makes them both DEF 13 with Poltergeist. Potential for POW 20 Essence Blast sprays.

Morghoul2 - Mortality. Can give them Incorporeal if they need to move through obstructions.

Xekaar - Mortality. Feat makes them DEF 13 ARM 23 in melee (ARM 25 with Agonizer).

Xerxis1 - Defender's Ward one one. Feat gives an extra damage die on melee attacks and +2 ARM.

Xerxis2 - Signs & Portents feat turn. Ignite for +2 melee damage.

Zaal1 - Between Last Stand and feat, a turtle can close out a game with four tail swings at 4 dice to hit and 4 dice damage. Also Inviolable Resolve for +2 ARM on one of them.

The problem is you are really greatly outranged by the turtle, the bane knights will never reach the turtle unless you have some really canny play. I don’t know Skorne so well, so I don’t know how much magic weapon they can hand out, but as I said earlier super Stalkers I believe out threat them and Wraith Engines should be able to get a good approach, even if the turtle is shooting at banes behind the Wraith Engines, eventually it will get caught, then the trick is to make sure you put it in the dirt, because at one box or full it kills the same amount of stuff, unlike a wounded beast/jack. Of course a cool way to try to deal would be zig one way to press the turtles and then zag the other way to kill their stupid warlock.

Against Rasheth, getting that ARM swing can be hard if has Castigate up. I can see A Black Industries list being able to crack the turtle’s ARM (although our ‘jacks are out-threated by it), but I can’t see a Scourge list, even under Asphyxious1 doing well. Under the Agonizer and Krea and feat, I’d think Satyxis would still struggle.

None of the turtle's attacks are AOE, so it's not going to give units as much trouble.

Animantarax armor: 20.
Average damage for Bane Warriors on a charge: 27 per model.
Average damage for Bane Knights on a charge: 24 per model.
Average number of Warriors to kill turtle: 3
Average number of Knights to kill turtle: 5

Delivering 1/3 or 1/2 of a unit isn't too difficult.  Dark Host has the 2 clouds to get them half way up the board, terrain is mostly irrelevant to banes, and our casters can cast spells such as Occultation or Veil of Mists to give stealth and block line of sight.  

Obviously, some things are going to die, but honestly, this doesn't seem like that bad of a fight.

Jacks are going to have a harder time due to them being bigger and fewer in number.  They get hit harder and you can't get as many of them next to the turtles, so they're going to struggle to kill it.

Scourge units don't hit very hard, so bad idea.

Infernal Machines could potentially get your Brute Thralls close enough to do work with Combo Strike.

Slaughter Fleet units are generally big and slow, but hit decently hard.  It would be an uphill battle, but doable with the right caster.

Ghost Fleet doesn't hit hard, but they regenerate decently, so have a good chance to engage turtles.

And let's not forget the debuff game.  Armor 20 isn't an issue.  I play against a lot of Khador and that's the standard.

Finally, there's always the option to completely ignore the turtles and just jam some fast units up their caster's rear end for the win...

Gamingdevil - Thanks!

Your math is off. Turtles have ARM 20 and 35 hit boxes. At 7 points of damage each, it takes 5 Bane Warriors on average dice to kill a turtle (or 9 Bane Knights at 4 points of damage each).

Even with Parasite, it still takes 4 Bane Warrior charges to kill a turtle (though 4 can do it even with sub-average dice rolls).

You'd need a 5 point ARM/STR swing (Skarre1 or Denny1 feat) to get down to 3 Bane Warriors killing a turtle on average dice.