Congrats man,
Dear all, as this thread shows, dont waste a zeta to unlock thrawn. There are so many much more powerful zetas than phoenix until you've played the game for 2 years.
If you cant beat this event without a zeta, as so many have proven in this and other threads, then you may be undergeared, have bad mods or the wrong strategy.
The simple strategy i used was having a potency cross on zeb, get good +protection on low gearrd tanks, auto first 2 waves, then use basics in wave 3 to be ready for wave 4. Then dispel taunts and kill deathtroopers first, then troopers, then thraen. I call zeb for assists to get stuns. Others call ezra. Gear 8 can do it, but the higher ezra is geared, the easier. Most use chopper and find chopper easier.