I used Bossk, Greef, Boba, Mando, and Fennec. All at R5 or higher. It took about ten attempts and finally Dash didn't AOE and wipe half or more of my team. If he just does a basic and then you can land an ability block on him with Boba, it should work for you. Fennec's basic does multiple attacks which gets to contract faster, then you disintegrate Dash, then Vandor, then it shouldn't be too rough after that. (I did have one come down to Han against my team and he kept one shotting them and I lost.)
Unfortunately, yet another event that is almost purely reliant on RNG. Nothing to do with skill, or with having your characters built up. Just plain RNG.