I love the logical extreme of Sion's dueling ability.
Sion has not demonstrated any real cutscene battle sequences, except for maybe cutting Kreia's hand off.
Sion has survived countless injuries, duels and such by sheer force of will.
Sion is one of three top Sith of his era.
Sion is a veteran of the Jedi Civil War and Great Sith War.
Sion is a talentless feeb that can be easily beaten by anybody; he just has regen that works like DnD trolls apparently.