After about two hours of constant attempts I was able to get a 7 star Chewy with the following

Gear 12 Boba Fett (Lead) - 224 speed / 44.9% potency
Gear 11 Dengar - 139 speed / 108.31% potency
Gear 11 Cad Bane - 137 speed / 101.37% potency
Gear 10 Greedo - 173 speed / 73.49% potency
Gear 10 IG88 - 145 speed / 85.48% potency

I was recording every attempt in case I was able to beat the event to help show how I was able to do it, but after two hours, I thought it was impossible for me. Due to this, I quit recording, but kept trying just for the heck of it. Three tries later I beat it, so I apologize for no video.

After the initial wave of attacks, I was left with Dengar in stealth (so R2D2 wasted his smokescreen), no ability block on Boba, my IG88 was defeated and Boba had lost Bounty Hunters Resolve, and Cad Bane was stunned.
I used Execute on Lando, which only got through his protection.
Dengar used Mini Mine Mayhem and landed 3-4 detonators on everyone
Greedo used Threaten on Han to remove all buffs and turn meter without him countering
Apparently someone evaded because Dengar went again and used Blast and Smash on Chewy, landing the stun.
I used Boba's basic on Lando killing him and regaining Bounty Hunters Resolve
The enemies continued attacking Cad Bane and killed him
At this point, I was granted the luck of the Gods because Greedo went next and used his basic on Chewy landing 6 consecutive hits
Dengar used his basic on him calling Boba to assist
Boba then used his basic killing Chewy
After that, Greedo then landed 5 more consecutive hits on Han
Boba used Execute on Han and killed him granting contracts to himself, Greedo, and Dengar
From this point on I just killed Leia and R2 and managed to win

I never thought it possible with my lineup, but it seems if the chances line up well enough, it can be done