Use Retribution to exclusively target Skye. With Discover, your entire team will be able to see Skye coming from a mile away. Combine with the item Keen Sights for added visibility
Lex is a thorn in the foot for Skye as she depends mostly on her stealth and when you track her it's more than easy to take her down with lex's insane damage normally it should only take up to two hits
When you are outside of stealth detection range but within the Retribution range, the insignia will still appear above Skye even though she is invisible. Shoot below it for expected results.