Without regards to your heroes available, Arachna is used both as a single and double tank. Which one depends on the makeup of the other team. If they have single damage like Slim and Garrick behind their tank, you need to protect her, often with an awakened Elric. If opposition does not have the damage dealers to take her down quickly, then she can single tank. Most often I pair her with Serafine to finish off the tanks and the midline once the tank is gone. Other team members to consider (situationally) are Cara, Gale, Mortus, Dokras, and Tholin. These are the ones seen most often with her as well. Dokras is needed if the other team has Mortus, Cara, and Slim as his rodents will often absorb their attack. Understand that Arachna is expected to die in almost every battle but is a fair trade for one or two tanks. If you do not take out their tank(s), you will lose. If your tank survives you will win. If both sides lose tanks (most common occurrence), then your other three will decide the outcome.