Sienna is not an anti armor class by design. In a good party, this is not her typical job. If Sienna is trying to hold down against armor too often, someone else isnt doing their job. You go in with a mace just in case you find yourself aggrod by armor in a bad situation, but its not advisable to go after armor specifically as sienna. Thats not her job. But that’s my opinion!
Sienna does best with good support, dishing out stagger and eradicating hordes, or effectively supporting front line classes. Sienna is not a front line class. Good sienna players can carry groups, yes, but is best operating as a ranged class dishing punishment at distance. The greatest of groups actually watch their sienna closely and keep one melee class like saltz or kruber or even kerillian/glaive stick close to the sienna kind of like a full time bodyguard. This allows sienna to really do what shes meant to… which is eradicate hordes, support front line melee, and catch aggressors at distance. Like a ranged tactician to soften some of the angles that give enemies advantage.
There are a few different ways she can be played, but even with unchained, being superb at melee against armor isnt her thing, its a weakness by design. Thats not what sienna is ever meant to do super well. Now if they suddenly gave her access to something like kerillians glaive, I would have a very different opinion because then she would be capable of much more effective handling of armor beyond how safely she can manage with just the mace. Mace can handle armor okay, yes, but it is not incredibly safe.