Söömo is a great support on every mission, including antag. However it's not as bad as you say (in my opinion as a raider). Sööma herself is quite helpless, her damage do weak damage and her ability doesn't help herself.
If she doesn't use a special card, all you need to do is break 3 protections in fast succesion, and she's dead from her ability.
You could focus on her. You could shoot that guy in the head twice.
Antagging in general has become quite hard, I don't think that Sööma is so overpowered compared to other raiders.
What does surprise me is that the shield completely negates one shot, no matter how powerful. I thought it would take away the damage, and if it breaks, leave the rest for the raider. That could be looked at in a future patch.
The antag has the advantage of seeing the raiders chosen. I've had tough Loath antags when going as Sööma.