Using aracnha as the tank in defense is a big mistake. Even in the mines the other day I got a couple of lvl 92 teams with awakened arachna tanks and I was able to defeat them in a matter of seconds with a strong single target physical team. The interesting thing here is, I am level 86! Yes, she did 100k+ damage to my group in 5 seconds, but she was also dead. My team had about 50% HP left, their team also had about 50% HP left except without a tank... The other heroes died quickly. Against a same level team it's not even funny, she doesn't get the chance to do anything.
In short, don't use arachna as a tank for defense, any smart opponent will destroy you fast. For offense you can use her as tank against some teams, like an opponent with Ezio on the front, or a non-DPS tank with low magical resist and not many strong single hitters in the team.