I think The Giant made a statement about optimization in the design of the defenses of each gate. Each relied upon the abilities of one class and all, save one so far, have failed.

TOotS being a diverse party with complimentary rather than overlapping abilities is repeatedly shown to be their strength.

Optimization is great against one type of attack. Chances are really good that the ten million other ways of attacking will be tried, especially if the DM hates Min/Maxing.