There is something I want to point out though. It is true that Sigma is a robot, and not entirely a living machine, but throughout the series we see that Reploids (Mavericks) have so sort of self-learning system inside, so they are not entirely machanical, either.

And I agree that Sigma will attack in a pattern, but it will be a more selective pattern. For instance, if the first time he attacks from Position X, when the pattern repeats itself, he doesn't have to attack from Position X, but Y, or wherever he chooses to. While he will use the same kind of attack, but there will be variations.

Sigma's advantage lies in...Seymour is mainly a spell-caster. Is it possible for Seymour to avoid everything while he is casting spell? Casting spell would take time, and it requires concentration. Of course, he can choose to fight in close range, but Sigma is very capable of melee battle himself.