I didnt mean literally. But i WAS taking it directly from AM5 p84 if you want to know what i was referring to...
Aren't horoscopes made for a place more than a creature? I thought this was why you could target someone/something you had zero knowledge about: you are checking what stars and planets influence the place where he is at the time. If you prepare a horoscope for a drake in Avignon and it flies to Jerusalem in the meantime, your horoscope becomes useless, right?
Yes, exactly. Magic moves the arm holding the sword. It's not the same as moving the rock above someone's head and letting go -- the "falling down" movement is natural. The sword's attack movement are not natural, if there were no magic to move it, it would just fall to the ground harmlessly.
I never kill my PCs. But that's an "incomplete contract" type that goes both ways: the players know I will (try to) set the obstacles according to their power level. But they also know that doing outrageously "wrong" stuff -- even if it's possible under the RAW -- will result in consequences.
Ars Magica is a game about consequences. Magi can practically do anything they would like, so the stories that arise from overly powerful magi are always "I could do that, but should I really?". And honestly, the only viable consequence for someone killing the archangel Michael is eternal damnation.
Again, Michael was just an example. Change the name to "Bob the faerie god", "Odin" or "Beelzebub" and there you go 
The fact that a Might 75 creature can be beaten at all is the problem with the system, regardless of its realm association. Might 75 should be impervious to hermetic magic not a mere annoyance to a young magus.
If a skellie with a sword cannot hit you, then a giant with a tree trunk (or a Gruagach transformed into giant size, for that matter) cannot hit you either? Or if you cast a spell on yourself to increase your strength your sword attack is stopped by Parma?
Xavi
Or at least bloody hard to affect by anyone lesser than a superpowerful old magi or a whole cadre of younger ones.
They are. Both reside beyond the Lunar Sphere, and are thus completely immune to Hermetic magic. The stats of saints are only given to represent their impact on the world, and are not their "real" stats. If an archangel will descend to earth, he will send an "aspect" of himself (of which he got infinity), so at best the magus has defeated an aspect - and will now need to face potentially infinitely many more, who are rather pissed, not to mention the fact that he hasn't achieved much.
Other angels and other Divine beings when they are on earth, now, yes - they're defeatable. I don't think that's bad, I do think it's too easy. It's especially unfortunate when it comes to more moderate but mythically-great powers, like dragons, faerie lords, forest spirits...
I'd note, however, that powerful Magical spirits at least are somewhat immune, too - only their aspects can be affected, and even summoning one requires a complex ritual. Likewise, while you can kill "a" Zeus in Elysium, can you kill Zeus? Still, these are not satisfactory solutions IMO; no one should be able to kill Zeus or Michael with a reasonable roll and a handful of raw vis, even if they are only killing an "aspect" of them.
I think part of the solution to all of this is greatly lowering the power-level of magi. But we've been there... 
I also despise multiple-casting with the burning intensity of a thousand suns (it's ugly, not mythic at all; the same goes for its sibling, fast-casting, when used for the same purpose), and think penetration-boosting methods (especially communion, raw vis, and item enchantment) should be kept in check. Even then, I'm not sure if doubling MR or more isn't suitable for at least some opponents, especially on their home turf - fighting a Faerie Lord or Forest Spirit within his demesne should be essentially impossible - he's the one setting the rules there.
Again, I agree. I hate being so clumsy as an SG/DM, I never let undefeatable forces work directly against the PCs, they only work through defeatable agents.
Plan B is not to roll in engagement between your players and the foes. The players roll, you do not. Basically because you do not have creatures with stats. I have done that on occasion and my players didn't nortice: I still rolled dice, but did not care at all about the result. (yes, Jordi, I know you read this board, so you can rant against my evil wickedness on Sunday). You compare the rolls of the players with themselves. If they roll well, they achieve things and you win if they roll badly. Quite easy, really. Tends to make BIG guys more threatening without having to work if their power is 30 or 2000. They just can do whatever you want them to do and grants that a stupid roll does not destroy the mythicness of the scene when facing Zeus.
Xavi
Its certainly a problematic "option". For some types of spells its very suitable, but for others its pure munchkinitis.
Again, i like my version as Might is subtracted from damage done you have to cast a spell of higher level than the foes Might, and combined with needing at least the same in penetration as well, and the damage is done to the might pool first with the foe being able to replenish it fairly easy by using might points, overall you need severely higher spells for the same foe.
To zero a might 10 creature with a single spell then needs a level 30 spell with 11+ penetration(and loss of Might never kills). Or multicasting 4 level 15 spells.
And i really do agree that "home turf" probably should benefit, perhaps simply a straight addition of the Aura*2 or *3 to Might? (or *10 if using RAW hehe)
Yeah, and while thats not entirely bad, it means some spell guidelines become exceptionally hard to reach, it means investing in Arts becomes dreadfully XP intensive and overall just becomes far too limited.
What, you mean casting The Shadow of Life Renewed might actually be impressive, and magi will have incentive to pursue relevant mysteries, use the various options (like ceremonial casting), seek out mystically-appropriate locations (like going to a suitably aligned Magical aura), and so on? How horrible. 