To be honest, even with invisibility, there is an Archer class servant. (And presence concealment isn't really invisibility. It's just that 99% of servants rely heavily on their natural ability to detect nearby servants that assassins just sneak on by. It's more like a fog of not paying attention.) 

So an Archer class servant could do a good job of detecting enemies. I'll try to post with Mehmed placing some shots into a group of chimeras.