Considering that there have been many cases where an active shooter came with multiple firearms, focusing on disarmament alone is very risky. (I realize that most posters here who say "disarm" probably intend to disarm first, then detain/disable.)
To me, the preferred solution in the scenario would be to disable the shooter WHILE preventing them from employing a weapon against you. Since I am often in places where weapons are banned, my best bet would be on multiple disorienting/disabling strikes, stabs, or slashes in quick succession (bonus points for getting the shooter off-balance and taking them to the ground). Ideally, this would be done from an angle that would provide good targets for strikes, etc and would surprise the shooter while being difficult to defend against.
From my, albeit limited, experience, keeping a sustained attack going severely reduces an opponent's ability to respond effectively. It's a lot tougher to fight back when taking hits.