I think regarding Seleucid armies as being rigid homogenous Greek armies of phalanx pikemen blundering about Iran is monstrously unfair. As Elfwine said, they would have been perfectly able to figure out that certain tactics favour certain terrain types. They specifically adopted the Kataphraktoi style of heavy cavalry, and let's not forget that in the 'Argead' style of phalax warfare cavalry is just as important as the phalanx itself is.
As for the survival of the dynasty, there's a number of things that can be done. Firstly, better control over the Parthian satrapy. Secondly, better control over the Bactrian satrapy. In the case of the former, the Parthians were the ones to prove the most threatening to the Seleucids; Romans did a lot of damage yes, but the Parthians were responsible for the most damaging territorial losses. In the case of the latter, the Bactrian satrapy was rich and densely populated, and when it rebelled the Empire lost a lot of resources that was then spent fighting nomads and conquering sections of Central Asia. Other things; not getting involved against the Romans is certainly a good idea. It's entirely possible that with a different King, on a different day, they might have been able to beat the Romans in a head on confrontation. Considering that the Seleucids were Greeks and Macedonians cut off from their homeland, controlling an Empire stretching from Anatolia to West India, they did a pretty decent job of attempting to consolidate it in my opinion in the circumstances. If that's the case, then I feel that the dynasty surviving is mostly going to be related to getting lucky in a few situations in which it was very unlucky.