First, let's expand the expressions:

1) (m^4-n^4)(m^4+n^4)
= m^8 + m^4n^4 - m^4n^4 - n^8
= m^8 - n^8

2) -m^6(m^2-n^2)
= -m^8 + m^6n^2

3) -m^6n^2

Now, let's combine the expressions:

(m^4-n^4)(m^4+n^4) - m^6(m^2-n^2) - m^6n^2
= (m^8 - n^8) - m^8 + m^6n^2 - m^6n^2
= m^8 - n^8 - m^8
= -n^8.