Legionnaires are one of the most common gladiator classes to be encountered in the game. They will be found in many Imperial leagues, and variations on Legionnaires such as Dark Legionnaires and Undead Legionnaires will also be found in other regions as well. They are a very well-rounded Medium class with no specific strengths but a very balanced set of attributes and some useful Innate skills.
A Legionnaire's most important abilities are their Innate skills. When fighting with other Legionnaires and a Centurion, they will gain a considerable boost to Initiative and Movement thanks to Practiced Maneuvers and Orders. On top of that, the skills Evasion, Indomitable Will, Discipline, Awareness, Riposte and Off Balance means that they are a defensive force to be reckoned with. Almost all status effects in the game will not affect Legionnaires who have taken these skills.
Other than that, they don't bring anything too unique or impressive to the table as far as skills go. Precision Attack is excellent for initially approaching opponents and perfect for attacking Light gladiators due to their generally high Defense. Bungle Enemy and Target Head can be used to lower an opponent's Accuracy and Initiative respectively, which is a plus in a one on one fight since they will get less turns and miss more often.
Legionnaires shouldn't be your main focus when deciding what gladiators to recruit for your school, unless your plan is to get at least two of them and a Centurion as well. There are plenty of other gladiators who are better suited for many situations than Legionnaires. Still, the best time to use one is against a Light class opponent.
When fighting against Legionnaires, use another Medium or a Heavy class to fight them, not a Light class. It doesn't matter how much Defense your Light gladiator has, Precision Attack will almost always hit and will deal extra damage to Light gladiators. If you're fighting a Legionnaire with a character who has low Defense, be wary of their Overhead Cleave, as it can cost you a helmet and deal considerable damage.