As for sacrifice driven rituals, they tend to have their own drawbacks.  Legal, ethical, etc.  If you're playing an evil version of the game where those don't matter, I'd make the Wardens a major threat.  If they're not already.  After all, power is detectable.

The problem with sacrifice driven rituals is it doesn't need to be other people.
Consider a group of five people. They have 5 mild consequences, between them. Probably a bit more from recovery powers, or extra mild consequences. A lore skill of 4, can probably make a couple declarations and its already a ritual of 20ish. Everyone throws in a moderate consequence and its at 40. Then you do a mass version of that healing spell (10 shifts, easy), and it will only take one more scene to get back to full health. I recommend going to a casino and trying to make some money from the other patrons in a social conflict. Or possibly an convention and trying a social conflict to make friends with your favorite actors via a social conflict. Or possibly a conversation convincing the wardens that you really didn't kill anyone for the sacrifice powered spell.

To your 4 methods
1) Mass sacrifice well this works per the rules pretty clearly, and easily if you can catch the 5 people. Worse, you can even sacrifice WVC, ghouls, or like as far as I can tell. And you can do similar by mass inflicting consequences.
2) I don't think 10 rabbits counts as a person, if they did no one would sacrifice anyone when they can buy stuff at the pet store. At best with sufficient rolls I would let declarations like ANIMAL SACRIFICE, LOTS OF ANIMAL SACRIFICE, ENDLESS BLOOD, BlOOD MAGIC, but those are really just declarations.
3) 50 declarations is hard. I know someone here made a system to adjudicate the difficulty and time requirements of declarations. Just use that, or similar.
4) It technically works, but at 4 scenes a game, your wizard comes back 25 games later. ... Its really not going to happen.

So most of these ways don't really work. Now how do you balance against big rituals? Well what can you even do with a 100 shift ritual that a 40ish shift can't? (Lore +5 Moderate +5 Mild + Change) I suppose you can help punch through wards and such, or make it area of effect to zot a city block, but really? A lot of baddies won't have wards. (No threshold). An aspect pile can be done just as well with 10 20ish shift rituals. What is seriously going to mess with a 30 strength ward?  Even a patently inane healing spell would top out before 40...

So whats the big problem? Its just Thaumaturgy doing what it does. If the PC's are really dedicated and have a symbolic link they can asplode the targets brain. If they really want they can aspect pile. So what is the problem going from 40ish to 100ish?

Yeah, I think the way to go about this is
1) Try to get a few taggable aspects before the fight. Assessments or declarations are good. Also are their other PC's that can help? They might be able to throw some in. If you have a thaumaturgist they can provide lost of aspects on you via thaumaturgy. 3 or 4 shifts of complexity per aspect? If everyone even just contributes minor consequences that is probably 3 or 4 aspects.   
2) Can you get any compels before the fight? Extra FP is always good.
2) WIN INITIATIVE! This guy will hit hard you might be able to take be able to survive a hit from him, but it will start giving consequences which he can tag and go down hill from their. If you can't win initiative do your best to avoid consequences. 
3) Shoot a 6 shift attack in his face. At 7 acc/6 weapon he takes an average of 11 stress. Unless you know he can't take extreme consequences 12 shifts is a good amount of damage to boost the damage to with tags. It forces him to take two consequences, giving you two tags. If he can't take extreme consequences tag aspects until you get 16 shifts of damage.
4) You should have FP, consequences and pre-set up aspects to dodge his next attack.
5) Finish him, he should only have 12 shifts worth of consequences left so a 16 shift of damage attack will kill him.