Thanks for all the posts guys!
Been using Vader, GMT, TFP, Jyn & Rex and it works well.
Done around 10 full solo's with this setup now and never get hit by Rancor apart from once or twice on phase 4, but with GMT intimidation tactics Rancor doesn't hit well.
Biggest surprise is GMT doing massive dmg (Vader culling blade type of dmg), if you hold ultimate fire power until the last 10% of each phase it can do 250k + dmg due of the stacking of his basic.
May swap in Krennic for Rex at some point in the future.