Normally you don't need Nihilus. They decimate their health by themselves enough. If it is a "weaker" Geo team you face, I would leave Nihilus out. Gives you more banners in GAC.
If you face a stronger squad, you have the possibility of putting Wat as third member in your squad ang give the tank tech on Sion if you don't need or want Wat in another squad. That way the Geos can't attack someone else and the risk of what you describe is a lot smaller. If Nihilus is not present in your squad they like to go for Traya unstead if they can ?
Edit for typo.
