i recently had a situation were a person attacked me were he sent maybe 20 pikemen with 3 captains, Here's basically his rutin, arrive, use Catapult Volley 3 times, retreat. He used this tactic a few times to take out my Ballista and Guard houses. Is this a legit strategy? or is it a total bit-ch move and they should nerf this?.

That's totally legit. Next up is catapult armies to smash to the keep. Removing the ballistas means catapults will do more damage because they're not being hit by ballista fire.

Yea, but what's even the point of having Ballistas if these sort of tactics is the norm? why should i donate more to my parish to upgrade ballista maker?

Just close your castle and put active pikemen into the exit, so no enemy troups can get over the white line and let the counter run down.... If the other player constantly loses his costly captains, he will stop doing that to your castles...

Not every body does that. And most certainly not the AI. If you want to fend off the ever increasing AI attacks you'll want more and more ballista in your castles.