Today I unlocked 7* Chewbacca with this team:
As you can see Dengar is only gear IX and Greedo is only gear X (was farming gear to put him on XI though).
I gave them my absolutely best mods, primary stats were all on health% or protection% where possible and the highest amount of speed in every mod. Before giving them my arena mods I couldn't survive more than 2 turns.
I used Bossk lead, had to reset many times until I got optimal start. Bossk was first, Boba Fett just behind him so neither of them could be stunned/ability blocked because after I used Predator Instincts I needed Boba to use special ability (Execute on Chewie to clear buffs). Then I focused Lando (and removed tm with Threaten), however I wasn't using any AoE that could fuel enemy tm. Greedo killed him with 3 criticals each about 12k. Then I went for Chewie, I was lucky that it just became the weakest enemy when it was Bossk's turn and I could use his middle attack to fulfill the contract. After that I used all my AoE to regen everything back and not being worried about Lando constantly using his AoE.
Chewie quickly died and contract went off. It was still 3v5. During the battle I lost Greedo to Han Solo but still managed to win with 4 bounty hunters left. Hope it helps somebody.