If a WIMP interacts elastically with a nucleus, for which force they interact? Weak force? The same for neutrons, it interacts via strong force? The only elastic scattering i know is the Rutherford scattering.
WIMP is an acronym of Weakly Interacting Massive Particles, so it is a tautology to say that their interactions are with a weak force. This could be the electroweak force, or any other in more advanced grand unified models, elastically or inelastically the first order, i.e. higher probability , Feynman diagrams are with the carriers of that force.
The neutron is not an elementary particle, it is composed of quarks and a sea of antiquarks quarks and gluons (analogous to the proton here) . If you are asking of WIMP neutron elastic scattering, it will be with the appropriate weak force. If you are asking neutron +X elatic scattering, it will depend of what X is. If it is another hadron composed out of quarks and antiquarks it will be the strong force.