How to counter mass siege units ?

Hi everyone !!

So today I was playing with my cousin I was playing as the Spanish and he was byzantines. The game was going long and he produced mass onagers and knights (we both are noobs ) He relies heavily on those types of units and it’s difficult to target siege when the cavalry is protecting them.

In the end I won but he obliterated my army, luckily I had a strong economy and spawn a lot of hussars and halbadiers when gold was running low and he decided to invade

I lost 1637 units and killed 547 of his.

HOW COULD I PREVENT A SLAUGHTER LIKE THAT if I already know he will always go for the same strategy?

Any help would be appreciated I really find this game interesting and I’m looking forward to improve

(Sorry for my English)