I've now beaten Vader once in about 15 tries. Did it by putting an offense set on Leia (although it could have just been RNG). Tenacity was not helping as Vader was still landing his DOTs. If you get to the point where Han is cleansing every turn, you're pretty much done as Vader heals whenever Han cleanses and the DOTs just get reapplied.
Still, GL events should basically be a story, not a chore. I'm not saying they should be super easy but the "hard" part about getting a GL should be farming the requirements. I have both Leia and Han at R7 which are higher than the requirements for the event. I'm not sure R5 would even cut it for this.