what stats i need as assist? and how much, about?

It depends on what you are doing.

I "know" three types of assists.

The classic AoE assist needs a lot of Stamina, and maybe adds a bit of strength and dexterity as well. There should be around 10 million people here that can tell you how much str/dex they prefer. Whether or not these still work later in the game I don't know.

Dex billposters (or dex ringmasters) are a bit like Blades --- they use quite a bit of dexterity.

The support assist puts everything into Intelligence (some add a bit of Stamina as well, but I don't do that); these aren't meant to kill anything, and pretty much suck at that. This is, obviously, the only real assist build (the others are normal combat builds based on the assist class, but they aren't assisting anyone). I will get an offensive DoT AoE spell at lvl 80 --- as of now, I have no idea whether that one will be effective (I'm not hanging out at lvl 80+ areas right now :-)), or even useable for me (if I hit multiple monsters with it then multiple monsters will start moving at me... which means I'm dead).

AoE assists are what you see all over the lower level parts of Flyff; so a lot of people come begging for plvl whenever they see an assist, even if you're a supporter. Later in the game, support and combat assists seem to be more distinct; I still get plvl beggars every once in a while, but for the most part I'm no longer a target for them.