Worth noting that Rider probably would have seen these attackers coming and Lancer would've had a good chance to depending on how they were made invisible. This doesn't seem like a fight that should be too dangerous even as an ambush, since if Masters can fight these chimeras Servants ought to cut through them like butter, but it's good to remember that 2 party members are more or less running see invis/truesight and there are likely things you'll want to let us know we can see, especially in the forest.