The Seleucid Empire:
Early start: You start with 6 regions(unless mods are involved). With their own garrisons. For ease of reference I'll refer to them by the name of their cities and not their official region name. Your first 3-5 turns should be you making buildings that increase trade and your temples. and increase your taxes. DO NOT DELETE ANY UNITS!! They're important for your survival.
Sardis Is by far your most important for your early expansion. From there your first goal is to take Halicarnassus. This should be done in the first few 3-5 turns as you recruit as many militia hoplites and mercenaries as you can. After conquering it, you rebuild your army and when you have what you feel as a comfortable amount of troops (half a full stack at least) Immediately take Rhodos and Pergamum from the greeks.
After the 5 turns of initial money making have passed start reinforcing Seleucia as the Parthians will attack you in 10 turns max. and see if you can take Palmyra before turn 15. Around this time Egypt will start attacking so you need to recruit as many things as you can and attack them first and take Sidon. You can let Salamis be the last if you want to as I always do so. You need push and take the Rebel town of Dumatha and Bostra before they're taken by the Partians and Egypt respectively.
By turn 30 the greeks will have called for a ceasefire as their war with the romans is in full swing but Pontus and Armenia will have joined in the pile of things that try to kill you. Pontus will attack Tarsus while Armenia will attack Hatra. Just putting militia hoplites in them will work to keep them in check. DO NOT RECRUIT Levy Hoplites as they suck against the missile troops of Armenia and Parthia have, while Pontus can be placed on hold while you deal with Egypt.
Overall Campaign Strategy . Be AGGRESSIVE! Seleucids while good at defense really shine on the offence. At least on campaign maps. Phalanxes work best on defense on battle map.
Once each city has their trade buildings and level one ports start making an army . You can start before if you have money to spare but your revenue takes priority.
You priority is to get the Greek cities out of Asia Minor then dealing with Egypt first followed by Parthia then by Armenia. Pontus is not a real threat except for their large stacks of crappy troops they attack with. Putting some militia hoplites and one or two Militia Cavalry should keep them at bay for a while if you're smart in using them.
Egypt is by far your biggest threat as they have rich areas and tons of troops and most importantly Early game chariots which means that if you allow them to advance too much they'll stomp on your pikemen.
Once Jerusalem and Bostra are taken, take an army from Seleucia, recruit some mercenaries and take Susa and hold it. Keep reinforcing Tarsus and Hatra as at this point you'll face a lot of attacks by Armenia and Pontus. Deal with those attacks but do not attack their settlements back unless you somehow lose a city. Overextending is a real issue.
Once you took Jerusalem you have choice to make. Either go for Petra or Memphis. I personally prefer taking Memphis and pushing for Thebes while keeping Petra under siege then taking it once I've stacked enough troops . Once Thebes is gone, make sure Memphis is well defended for when the Egyptians try to take it back then once their attacks have been repelled, take as many troops from Memphis and Thebes as you can without them rioting and lay siege on Alexandria. Once Alexandria is gone, build a ship and take Salamis with some of your troops from Antioch. and then push from Susa from Arsakia at the same time. Once Arsakia is yours go for the closest Armenian cities and slowly push the Pontus back with Troops from Hatra, Sardis and Pergammum.
DO NOT EMPTY Rhodes!!! Your first target when facing Pontus must be Nicomedia as the pass to Tarsus is too well defended to break through easily and Ancyra is very well defended Once you've taken Nicomedia and are pushing for Ancyra, immediately push for Mazaka. Ignore Tribus Sakae as it is too small to actually bother you for a long long time. Once Pontus is dead or while you're mopping up with their last city Numidia will attack. you need to push them back and head to Greece and take their wonder ASAP. It give happiness bonus and at this point you'll have a lot of culture penalty. Taking kydonia first is optional but not recommended as you need to push for mainland greece first which should be in the middle of a Brutti invasion.
IF YOU CAN BUILD SOMETHING IN A CITY THAT IS RELATED TO IT'S MONEY MAKING OR HAPPINESS THEN BUILD IT!!!!
At the greek invasion time you should be able to at least make elephants and Phalanx pikemen. Beware of Spartans if they still have them. The damn things mow down through all militias and Levies you have and Phalanx Pikemen are only good in 3 to 1 odds(make that 5 to 1 to be sure). From here on it's a slog against the Numidians and the Romans and whatever barbarians are left. Except the roman nothing should really bother you until you reach the Britons(if you're aiming for full conquest).
Overall battle strategy. Pikemen form the core of your lines. Levy pikemen are not to be left anywhere near missile troops as they'll get routed almost immediately due to their poor defense. A good pikemen core and a decent number of missiles should do you well.
Militia cavalry are good missile troops and can be used in any stage of the game against light infantry and even enemy cavalry and missile units.. Greek cavalry in general is shitty but Cataphracts and Elephants the Seleucids have access to are good. I've never used cavalry (except missile cavalry) in anything but a support role in any of my campaigns as they're pretty much trumped by any spear users. Hell even your general will die to the crappiest spear users. They do have their uses in dealing with Missile troops and enemy cavalry as well as non spear units however so you should not neglect them.
Elephants are not to be used in the front as bullet sponges but to raise hell as your infantry is keeping the enemy busy. The losses followed by the demoralising effect elephants have should make the enemy infantry start to rout on the open field
When defending cities phalanxes are you best friends as you can use them to create choke points that will make your enemy pay dearly for every inch they take. My personal strategy is to place 3 pikemen units at an entrance when the enemy has taken down a wall and wait for them to die while my units are in phalanx mode.