I would wait to champ Kitty as Cable is not the best partner for her. He could work, but you would need to race to 9 Yellow AP in order to get out some Strikes. If the required 3*/4* makes specials, then it's better. But I've found that a half-covered half-leveled Kitty works great in PVE. Yours is probably buffing for around 300 each turn, so combined with Grocket and Medusa / Thanos, you could blaze through SCL 8 and rank top 10, maybe top 5. Nevertheless, you could champ Kitty alone and play the Kitty-Grocket combo in PVP. A friend of mine did that and he didn't have any problems with it. There should be enough single 5* champ teams to queue. Even if you face Thorkoye teams, they are easy to beat as you naturally collect their colors anyway. Just make sure you save up some hits to finish off Thor before his 1/2 health passive kicks in.