Would this solution be sufficient?
f is entire, then it has a Taylor representation about 0:
f(z) = a0 + a1*z + a2*z^2 + ...
Also, we have lim z-> infinity f(z)/z = 0, i.e. we must have a1, a2, ... = 0
thus f(z) = a0, a constant.
I used some reasoning from real analysis, can this be...