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I imagine this will be like the fight between Khan and Mortarion. Khan notes how slow Morty is, how he's literally able to dance around him, but the damn big guy just won't go down no matter how much he slices. It doesn't even phase him. Mortarion, save for perhaps Ferrus, was unmatched in his physical strength and endurance. Khan thought that they complemented each others weaknesses, and no force in the galaxy could stand up to both of them.
Fulgrim is fast, graceful, his every move perfect and his skill with a blade unparalleled. But TalOS is a bag of dark age tricks wrapped up in the form of a tank and spewing fire and plasma. A knight fighting a fire breathing dragon, he's gonna need a better sword than whatever he's got now. You cut into him, and nothing actually bleeds. There's nothing beneath the armor, he IS the armor.
Also probably what helps a ton is just deploying a few tricks we've learned from Russ, that's probably the biggest thing TalOS has going for him. His capacity for knowledge, combined with an enhanced body surpassing his original primarch limitations.
