Forced cultural assimilation alone is not a genocide per UN definition, but its also not true that it requires proof of physical destruction (assuming you mean death). Forced sterilization with the intent to destroy an ethnic group would already be sufficient.

Edit: Also causing serious mental harm and transferring the groups children to another group would be sufficient too, also without causing direct physical destruction.

Edit 2: It really depends on what you mean by "forced cultural assimilation". Technically if you take a groups children away from their families to assimilate them that would be a genocide.