  We find a model-independent upper bound on the strong coupling scale for a
massive spin-2 particle coupled to Einstein gravity. Our approach is to
directly construct tree-level scattering amplitudes for these degrees of
freedom and use them to find the maximum scale of perturbative unitarity
violation. The highest scale is $\Lambda_3=\left(m^2M_P\right)^{1/3}$, which is
saturated by ghost-free bigravity. The strong coupling scale can be further
raised to $M_P$ if the kinetic term for one particle has the wrong sign, which
uniquely gives the amplitudes of quadratic curvature gravity. We also discuss
the generalization to massive higher-spin particles coupled to gravity.
