Notes #02:
Some more observations regarding the issue of Archer's hounds, and somethings that may apply to all Servants as well:
• On the issue of them tracking people while in Spiritual Form: please, bear in mind that, while in Spiritual Form, a being can only perceive spiritual sensations. In other words, they can feel the presence, and a vague notion of distance and maybe (but not always) direction to other nearby spiritual beings. They can't see, hear, or feel anything, nor interact with the physical world at all. If they, or any Servant, want perceive anything physical, they have to shift back into a physical form.
• On the issue of them trying to appear as normal hounds, suppressing their presence, while in physical form: they cannot. That would require having the skill Presence Concealment, which they don't possess. On that matter, even while in Spirit Form, they can be perceived by other Spiritual Beings and even magi as long as those have the means to do so (usually Spiritual Evocation users, or those with appropriate Sorcery Traits, like specialized Mystic Eyes).
• On the issue of their durability: having around C Rank END affords them a decent amount of durability. However, if they are hit with a clean blow from someone like Saber, Berserker, and Rider, they are going to die instantly. A single Anti-Army Noble Phantasm, or Area of Effect spell can wipe out several of them at once as well.
On the matter of spells, even though they have a minor (D Rank) Magic Resistance, they can still be hurt by and suffer the effects of magic, so beware about launching them against Masters carelessly, since they are almost more than capable of killing one of these hounds with their spells. D Rank MR might make it very hard for pretty much all of them to hurt these hounds, since it cancels most spells that are generally useful in combat, but at least Alexa can easily one-shot one of them thanks to her High Speed Incantations. A spell with a chant of Three Lines or more should be able to kill them in one hit, which hard for most mages to pull in the middle of a battle, but Alexa can do in One Line (or less if she has her Mystic Codes with her).
The same is valid for Servants as well. Magi may be much, much weaker than Servants, but if you underestimate them, you are gonna end up in a pickle.