Jaina was no weakling. Whatever they did, there was no way to take her down in one fell swoop.
If they attacked head-on, even if Jaina could not defeat them, she could hold out for a while. That would be enough time for reinforcements to arrive.
The best way to subdue Jaina in an instant was a sudden ambush. But ambushing her simply would not work.
They were not members of some other race. They were Undead. In theory, the aura of the Undead could be concealed, but only from those who were not particularly strong. Concealing their aura from a Heroic Powerhouse was no easy feat. If the Undead approached Jaina, she would detect them from afar. Ultimately, it came down to one thing: the Undead had existed for far too short a time. How many years had it been? Their development lacked the accumulated knowledge of a long history, leaving them deficient in many areas.
If the Undead had a history stretching back several hundred years, they would surely have developed a method for concealing their aura. Then approaching Jaina would no longer be impossible.
Every powerful fighter the Undead could deploy was steeped in that aura. An ambush was impossible, and so was a frontal assault. The best approach, then, was to avoid Jaina and seize an opportunity to act.
If they wanted to rescue Neptulon the Tidehunter this time, the part Rex himself had to play would come in the middle of the operation.
If fighting Jaina would not work, then they would avoid a fight.
Having Jaina transferred elsewhere was unlikely, but finding a way to get her out of the way was certainly possible. If something came up at the time and delayed her, making her arrive late, that would serve their purposes perfectly.