I will be honest, I find that 6th edition Terradons are awful against any war machine army. I realize that is what they are supposedly meant to take out, but they are terrible at it.
A unit of 3 Terradons has 6 T3 wounds. Off the top of my head, I will say Empire, Dwarves, High Elves, Dark Elves, Skaven, Orcs + Goblins and Tomb Kings have war machines often enough that you have to worry about it.
I have yet to see a Dwarf or Empire line without an Organ Gun or Helblaster, which both have a range of 24''. As the Terradons are the biggest threat, they will eliminate our poor flyers before the Terradons get a chance to charge (barring a lucky misfire). Without either we have a much better chance of course.
High Elves and Dark Elves are even worse due to the Repeater Bolt Thrower multi-shot, which will on average kill one Terradon outright. If there are two Bolt Throwers or any sort of move-and-fire shooting troops (Dark Riders, Repeater Xbow, Archers, Reavers, etc), the whole unit will be wiped out most likely, or fail their Ld 5 cold-blooded Panic test. Skaven usually have Jezzails and Ratling guns, the former may need to shoot a Stegadon or something, but the latter will wipe out the Terradons, and we can't even charge it thanks to it being able to stand + shoot. At least the Warp Lightning Cannon has to flee. Tomb Kings might be able to kill the Terradons with a doubletap on their archer unit. Orcs + Goblins is really the only army that Terradons will have an easy time wiping out the machines (and triggering some fanatics, fun stuff!).
Wood Elves, you might as well take the Terradons off the board immediately.
With the new rules, we can at least use Woods as cover from a lot of shooting, so it is going to be a little easier to charge safely. However, we lost the -1 for US 1 Skirmishers as we are now Flying Cavalry, so we are just as well protected in woods now as we used to be out in the open...
To boot, Terradons are much weaker in CC then they used to be. I mean, the Skinks HAVE spears on the models (even the NEW models), couldn't they have given them spears in the list?
Don't even think about dropping rocks on the machine as you fly over. 2/3 rocks will hit the machine (and wound on 6's), so its likely to be a waste of time. They should be used on Fast Cav or a missile unit that can panic easily and has a low AS.
At least you need only the 3 models to crossfire people now (as opposed to 5). As well, they are LD 6 and cheaper by a bit.
I still use my Terradons because I love the models but it is not for no reason that I call them Terribledons
As for anti-war machine in the new book...the best I can suggest is Commandment of Brass or Rain Lord (if you bring a Slann). Even Tik'Taqto and his optional unit will have to take one round of Organ Gun/Helblaster/Bolt Thrower to the face before they get to charge, though they have a better chance of surviving and/or passing panic. Chameleon Skinks are hardly an option....