If you combine all Ewoks, they are lots and lots better than on their own. I currently have Teebo at 5 stars, Chirpa at 4 and Elder & Scout at 3. Combined with RG or another tank they are already becoming quite good in GW because of their amazing synergy.
Because of their turn meter manipulation and various buffs specifically for Ewoks, I'd recommend farming the other Ewoks as well as Teebo if you want to use him. (Long, slow, painful grind)
https://youtube.com/watch?v=EcY4GdYSpw8&nohtml5=FalseHere's a video of Ewoks in high tier arena (RJ_Brando's vid)