Learn to use your arachna. She is uber strong in arena. Here she should be able to single tank since there are not strong single target heroes on the other team (understand that IF there are (like Slim), you need a real tank to protect her). So single tank means she is the only tank and will be in the front. Goal with her is to take down Leon. Then she will die, having done her job. Ethera is the big threat. If she ults, you lose. So back arachna with Serafine, Mortus, Cara, and either Gale or Garrick.
Lose? Did Arachna kill Leon first? If not, then add Elric or Leon or Drago instead of Serafine or Gale or Garrick. This is the double tank (with arachna) being one of them). These two variations will be effective against many arena defenses. Learn to recognize when to use one or two tanks and which heroes to switch out and when to use others (including Dokras, Tholin, and Slim) in place of one of them.