Yeah, with that roster, or really just with that Krisk, you should definitely be able to do more on CB... are you using Aothar? Poison is by far the easiest way to increase your damage numbers on the CB - that and warmaster/giantslayer, the top level masteries. Maybe a team with Krisk, Athel, Apo, Aothar and Mistress? Although I'm not sure how well Krisk can keep the decrease def and decrease atk on the CB by himself. 

Struggling in the arena is normal unfortunately, everybody is struggling. Although you might have the champions to build a defensive team that simply withstands speed nukes instead of trying to outspeed them... would be interesting to try something like Krisk, MM, Mistress (lead) and Apo. At least against speed nuke teams that don't have much healing/sustainability themselves. 

I'm not sure Mistress is really so great to use in the long term, but with her resurrection and heals, she should be useful support for you now, plus a very solid aura (which, unlike Apo's, works also for Clan Boss).