Whoa you people must be really skilled to beat both of them, that early.
There is a ladder that prevents the cutscene from happening so you can clear out all of Fort Joy, so long as you don't leave for the Marsh.
By then, you can fight them with level 5 characters and somewhat okayish gear and most skills.
Nevertheless, Dallis one shot anyone she reaches in melee and Alexander will kill anyone besides high Physical Armor characters in a single Battle Stomp.
So the only way to win the fight is by keeping Dallis away with Teleports and either laying an ice field in the gate to Fort Joy to get some knockdowns on passage or leaving a cheesy static cloud or puddle that she will refuse to cross. Meanwhile, you let Alexander cross the gate and come to you then lay down enough DPS to remove his Physical Armor so you can Knock him down on the first round and get him low enough on the second that Dallis will *redacted* (I won't spoil that part, let's just say Dallis and Alexander together leave the fight if one of them gets too low.) leaving only her 2 pets and the couple of magisters. Being level 5 yourself, the level 2-3 magisters are nothing more than flies waiting to be squashed by you and the 2 pets won't do enough to kill you if your party is capable of the feat of beating Dallis and Alexander.