It isn't based on any tricks. You get nin out of luck in this kinda treasure, in the little beginner's 20 thingy, you're guranteed a rare at 3 different numbers, and they likely got Sasori on one of them. Level doesn't have anything to really do with this, sure, it may limit some nin you get, but hardly.

Heck, I didn't get my Sasori until like...my 70's, if not even higher then that because I had to piece him together before he was in shops, before the beginners recruit (Got Tobi anyways as my first Seal rare :3). But the point is there is no trick to getting people, some people that are 20 levels lower then me have nin I don't have, which I think 1/2 of the people reading this should know exactly who I mean.

But the point is, well..actually, don't focus on getting him by drawing, wasting seals that way won't benefit you the most in the long run. Even though I do motivate fun, most of the nin there are really easy to get in modern gameplay, Sasori's in the ranked shop, you just have to wait.