Really he had already gone wrong before the fight started. Estrella will judge a target in Robaldo's position as vulnerable because she can't see what magics he has readied. A regular old Detect Magic would work, but anyone who gets seen using it is raising alarm bells, and she needs the surprise.
There aren't any good ways he can think of to give Estrella the ability, and even if she had it she wouldn't be able to interpret the results. Really he needs to be sending two subordinates, and not just one, but Estrella is his best minion by a considerable margin and he's only got the one pair of boots and if they know anything there's a risk of information leaks.
If he'd at least been on a Telepathic bond he could have tried to interpret a description of the target, but the description wouldn't've been much better than "is dressed weird and has a mud buddy who is dressed weird" and if that's enough to scare him off he can never kill anybody.
New idea: Have Estrella visit target location a few days in advance to scout. While there, she picks a suitable victim who isn't powerful enough to resist and steals some hairs. On the actual day, he'll use them to target a Greater Scrying right before the mission, and tell Estrella their location. Estrella picks up the victim and Dominates them, and orders them to be Stristyko's eyes. He can then cast detect magic through the victim, and Message to let Estrella know if there's anything that changes the strategy. He can't cast any illusions that would hide the scry from the target, but there's probably some plausible excuse.
He's not going to trust the idea yet. He's an idiot, it's an idiot's idea.
He'll test it on a softer target a month or two from now.
Really Stristyko wishes his assassinations failed more often, Tet's cleric has taught him a lot.