Yeah, for a complete victory 1 on 1 you have to encircle the enemy, using tight formation, and get as close to the enemy as possible (that means high casuality and friendly fire). It's also easy to lose the control of your troops once the encirclement is broken. But when you have equal or higher numbers (of solders, not of units), you can always win.
The problem against seleucid is not pike's heavy armour, but hypasists' fast speed and javelins. It's most difficult to fight against heavy armoured javelin/pila-thrown infantry, because you have to keep your units moving all the time, while getting them close to the enemy, otherwise the arrows would be totally useless.
A new tactic I used is to fight the same army by at least 2 battles instead of 1. First you make one or two big, tight squares, turn off skirmisher mode, and run it from the front of the enemy to their right flank, and to the rear, then retreat. The massive arrows from right side are usually enough to destroy their light units immediately, including the hypasists, and maybe cavalry too (ex: hetairoi). And then in next turn, when they have only the heavy & slow pikes, you can easily encircle and destroy them. Charging may also be used when the enemy is shaken or weavering - just be careful about the friendly fire.
And yeah, the seleucid have virtually unlimited money, like all other big factions - because units are produced too slowly in RTR

. That's why you should never try to confront their armies - it'd cost a lot and if you win you just make seleucid an easy target for other factions.