Does shifter flurry gives iterative attacks at 11 and 15 if player crossclass out of the shifter class or does this ability requires 16 level of shifter to work at all. Do get me that I'm desperately trying to find a viable build out of 20 level adaptive shifter right now preferably with base shifter so I'm looking at ways of using the flurry.
I presume you mean Shifter's Fury? That's tied to Shifter levels, and not BAB like actual iterative attacks.
If you want a strong Shifter build, you need to use a form with pounce, anyway, and those have three primary natural attacks*. Shifter's Fury just isn't worth it for those, which in turn means that after 9th level, you can multiclass out of Shifter without loosing much.
*) With the Quillcat (incidentally the only small animal with pounce) being the sole exception.