Ethera counters ethera with luck. As for arachna I just kill her with my physicals then heal next round. Replace Riley by Garrick in your second team will help too
Previously I had never liked Garrick in CoF but will give him a try and see how he works out. Thanks.
With Ethera I find it really annoying when you start a battle and she immediatly ultimates and decimates your team leaving it very hard to recover
Ethera is the biggest problem in CoF because of how quickly she ults. You might try to keep a charged Mortus and Slim in reserve for this. Hit with charged Gale or Jas right at the start of battle. Then Mortus and Slim. Sometimes they will take Ethera out before she ults. If they target another hero, exit and restart.
Second option is to have some heroes that will survive her ult or can be sacrificed. Gizmo and Dokras are a place to start.