Wilhelm went easy on him on that Omake.
He could've picked Paradox-Billiards-Vostroyan-Roulette-Fourth Dimensional-Hypercube-Chess-Strip Poker
-but is suppose not even the crime of incinerating all of humanity would be worthy of such torment.
Wilhelm has grown complacent, he doesn't train anymore. Doesn't bother learning new magic, nothing. Maybe he does but you don't write it but atleast explore more magic conceptually, he can fight God's but it's not his strength.