"highly stealthy" is a correct and usable phrase in written English.
You can use it to describe something that is very difficult to detect. For example: The new spy drone is highly stealthy, making it difficult for the enemy to detect it.
These stealthy, highly maneuverable jets have flown thousands of sorties in the global war on terrorism.
To enable the study, autonomous, stealthy, and highly maneuverable underwater vehicles are highly desirable in monitoring of the invasive species.
"Supporters of terrorism are stealthy and highly communicative and we need access to this data," said Wolfgang Bosbach, the Christian Democratic Union chairman of the Parliament's internal affairs committee.
Inherently stealthy and highly tumor-selective gold nanoraspberries for photothermal cancer therapy.
The RQ170 Sentinel is a stealthy and highly capable unmanned aircraft.
N. Gandra, Portz, C., Nergiz, S. Z., Fales, A., Vo-Dinh, T., and Singamaneni, S., "Inherently Stealthy and Highly Tumor-Selective Gold Nanoraspberries for Photothermal Cancer Therapy, Scientific Reportss, 2015.
Although substantial research has been done towards botnet detection, it is becoming much more difficult today, especially for highly polymorphic, intelligent and stealthy modern botnets.
As discussed in Section 1, the three characteristics in modern botnet, namely highly polymorphic, intelligent and stealthy, make current botnet detection approaches miscellaneous and manifold.
When we are truly focused on this issue, a depressingly rare occurrence indeed, we are playing by an arcane set of rules in the face of a highly sophisticated, totally committed, stealthy, deadly, hydra-headed opponent who knows no rules of engagement.
