Notwithstanding any provision of law or actuarial rule, beginning in the calendar quarter of the year that a tax is first imposed under s. 77.701 (1), in any retirement system of a 1st class city, the required annual employer contribution shall be calculated using a 30-year amortization period and an annual investment return assumption that is the same as or less than the annual investment return assumption used by the Wisconsin Retirement System for participating employees, as defined in s. 40.02 (46). Future unfunded actuarial accrued liability due to factors such as market returns and standard actuarial practices may be amortized on the basis of standard actuarial practices. The amortization period and investment return assumptions in this section shall supersede any amortization period and investment return assumption adopted by the actuary or retirement board of the retirement system of the city. No trustee or administrator of a retirement system of a 1st class city shall be subject to liability for complying with this section.