So, on sunday I played a game with my brand new unit of finecast Terradons. I had never used them before, and they were very fun. I actually played two games, one against ogres, and one against skaven, and used terradons in both games. In both games I got to use Drop Rocks, and I had targetted a war machine both times. Against the ogres, I targetted thier cannon/chariot monster, and against the skaven I targetted a Warp Stone Cannon. Both times, the drop rocks did NOTHING. The terradons still performed very well aside from the drop rocks, but those pesky rock drops did not work out well for me. I got 6 hits (out of 3 terradons) against the ogre warmachine, and I got 9! hits against the warp stone cannon. I still didn't do ANY wounds though. The high toughness of the warmachines saved them each time. So my question is; is it better to target a war machine with drop rocks, as I did, and just hope you roll some 6's and 5's, or is drop rocks better used against a unit of core or something where S5 hits would be more effective?? Also, what do you usually target with your "drop rocks"?? I'm thinking next games I may target units with the drop rocks, fly past the enemy line, and then use the terradons to attack the warmachines from behind, and rely on poison to help bring down the war machines, instead of the Drop Rocks special move.
If no other targets present themselves, I will drop thim on warmachines. Usually, I try to target medium toughness targets and hope to do enough wounds to get a panic check. Warmachines are best handled by the chameleons or skink skirmishers. Usually, what I do with my terradons is fly over stuff, drop my rocks, then shoot with them. Follow that up with a charge on something preferably in the back field. I usually will also take a skink chief on a terradon to add some extra power to their shooting/close combat attacks. Waaaay back in 7th edition, I would take the staff of the sun for the shooting attacks it grants, but it is not worth it as every single time that I took the darn thing, my other terradon riders out shot the chief.
The drop rocks are only S4 so they are best used on squishy (T3 or T4) targets that are of high value with little or no armour save. Some excellent targets for them are missile troops, fast cavalry, lone wizards foolishly outside of units, weapon teams, etc. Relying on 6s to wound with rocks will inevitably disappoint but their poison javelins do work well as Arli has indicated. The idea of shooting up a war machine first and then charging in will work against most war machines since the crews are T3/I3/S3, allowing the terradons to go first and kill the crew before they even get to strike back. Be careful however as some "war machines" like the hell cannon can crush your fliers with ease.
I drop rocks usually on whatever will be killed outright and then cause the most panic tests (in the movement phase noless) this means that anything nearby can also be targetted in the magic and shooting phases for additional panic tests. The best target for rocks are stupidly placed chaff units, for example sabretusks. Killing one of these mutts and seeing a bull unit run for the hills is priceless.
I personally tend to try and go for the hard to hit (and preferably expensive) units like skirmishers (e.g. skaven gutter runners or dark elf shades)- this uses the advantage of the rocks hitting automatically. Ideally, it'd be great if you can drop the rocks and then line up a charge at a warmachine for next turn. Against ogres, (as MOMUS already said) I'd go for anything which has the best chance of causing panic tests- ogres are notorious for failing panic early in the game (well, in games I've played anyway )
I'd throw my vote in for squishy things, war machines seem like a waste to me as they have toughness 6 or 7 against a shooting attack. 3 Terradons, D3 each will probably give you 6 which will wound on a 6. Skirmishers (and maybe troops garrisoned or behind cover) on the other hand would be very nice given that you get D3 hits so don't have to worry about to hit modifiers. On the other hand a skirmished unit at T3 would be wounding on 3+
drop rocks is a small rule that terradons have pretty much "just for fun". They arent likely to make much difference in any fight. The one thing they CAN do is to hurt mages who are foolish enough to stray from their units but who are out of sight (so you cant charge them). you arent likely to see an opponent be foolish enough to let his mages get killed by this, but that will only be because he will be forced to stay with them inside units, units that can be attacked in CC and killed. Hence the drop rocks primary purpose is to force the opponent to act in a certain way. they rarely do any real damage in the form of wounds, but they do cause damage in the form of limiting your oponents options. Once your opponent stops thinking about how he want to act, and start reacting to how you act, half the battle is won