Seleucids' best weapons - Lessons from the Numidian campaigns
It is true the Seleucids have one of the most interesting choice of troops. But elephants, chariots, cataphracts, silver shield pikemen and silver shield legions are not really needed to win an imperial campaign, at least not on M/M. We might want to use these crack troops just for the fun of seeing them in action (being on the giving end of a chariot charge makes me feel good :-))
The Seleucids already have 3 very powerful "weapons" early in the game, before getting access to these super-units:
1. Money
The Seleucids make so much money than they can probably win the campaign without having to fight a single battle. For instance, Halicarnassus can be conquered by bribing the large rebel army outside its walls, which results in keeping the hoplite militia, then bribing the city garrison and moving the newly acquiered hoplites in the city, to prevent revolts. Sidon and Jerusalem fall in a similar way - bribe the rather large Egyptian armies in the area (get one family member in the process) and then bribe the garrisons and move some of our troops in. As a Romanian saying goes, "a golden borer makes holes in any city wall". The same with Pergamum, with the added benefit of keeping the hoplite militia and the family member. I think the ideal Seleucid invasion army is made of a general travelling together with a spy (for avoiding ambushes), an assasin (for added protection) and a diplomat. The diplomat bribes the garrison and the general occupies it with the mercenaries recruited on the spot. I'm currently sending such an army from Antiochia, to invade the British isles. I'll let you know if this really works. If it does, the Senate is in for a big surprise!
2. Temples of Hephestus
They improve the weapons and armor. Combining it with the blacksmit family of buildings results in having troops with extremly good stats very early in the game.
3. Militia cavalry and peltasts
Well, this is like playing a Numidian campaign, but with stronger units (due to the combination temple of Hephestus + blacksmith/armourer/foundry and slightly better stats of the militia cavalry).
The Numidians don't have the money to afford weapon&armour enhancing buildings early on and their starting cities grow too slow. The Seleucids don't have these problems. They control the Hanging Gardens, which means their farms produce lots of food, which helps population grow. They can further boost the population growth by enslaving. Asia Minor is full of cities close to each other, presenting the conqueror/slave trader with plenty of opportunities early on. Since they have the money needed to bribe other factions' family members, Seleucids can have a governor in each of their cities. This means slave convoys will travel to the remotest corner of the empire. And a horde of militia cavalry, crushing one enemy army after another can turn captains into generals as quickly as this happens in the Numidian games (where roughly 20% of the family members I have are brave captains who rose through the ranks).