Using Nightmare:
Debuff them all with Vader
When Thrawn's turn comes, swap TM with Nihilus. Drain Force.
Then dispel zombie with Sion
Fracture Talzin, shock Zombie.
Then just get shock on them and focus them down to less than 20% health each. Don't kill them, if oyu do that then the first 2 times you do it they'll just revive. Let shock + DoT kill them on their own turn, they won't revive that way.
Use Vader to keep them ability blocked so they don't use specials.
Annihilate with Nihilus.
By the time you've gotten to Annihilate normally at least 1 of the other NS has died from DoTs, so there's just 2 + zombie left. Easy peasy. I've done it several times without even getting plagued.