We start with the expression:
\[
(8 x^{6} - 8 x^{4} + 12) - (4 x^{6} + 13 x^{4} - 8)
\]
Distributing the negative sign through the second set of parentheses gives us:
\[
8 x^{6} - 8 x^{4} + 12 - 4 x^{6} - 13 x^{4} + 8
\]
Step 2: Combine Like Terms
Next, we combine the like terms. For the \( x^{6} \) terms:
\[
8 x^{6} - 4 x^{6} = 4 x^{6}
\]
For the \( x^{4} \) terms:
\[
-8 x^{4} - 13 x^{4} = -21 x^{4}
\]
And for the constant terms:
\[
12 + 8 = 20
\]
Step 3: Write the Simplified Expression
Putting it all together, we obtain the simplified expression:
\[
4 x^{6} - 21 x^{4} + 20
\]