已知函数\(f(x)=\cos ^{2}x+ \sqrt {3}\sin (π-x)\cos (π+x)- \dfrac {1}{2}\)
\((\)Ⅰ\()\)求函数\(f(x)\)在\([0,π]\)的单调递减区间;
\((\)Ⅱ\()\)在锐角\(\triangle ABC\)中,内角\(A\),\(B\),\(C\),的对边分别为\(a\),\(b\),\(c\),已知\(f(A)=-1\),\(a=2\),\(b\sin C=a\sin A\),求\(\triangle ABC\)的面积.