Basically there are rules for casting a ritual spell but not for a group to magically resist said antagonist group. Or to teamwork their opposition to a hefty spell.
SO the basic everyone throw dispell dice at the problem works but would there be an advantage/restrictions on the mode of ritual dispelling?
The parameters I am looking at is there is a group seeking to stop a toxic shaman's sacraficing fueled ritual of doom TM. But they don't know where the ritual is taking place specifically, only a general area of downtown Seattle.
What would your surggestions be and discuss.

Do they have any information about the toxic group? If so, see if you could either (a) find them with a spirit's Search power or (b) target them with your own ritual first (see street magic for a variety of options that can mitigate the need for LoS with respect to ritual spellcasting), or © determine the appropriate target and use their ritual link to track them back to the ritual's source and whack the team before they complete the ritual. Those are all within the rules of the books.

Also, dispelling only works on sustained spells, so if the toxic's Sacrificing-fueled Ritual of Doom ™ isn't to produce a sustained effect, then they can't really dispell it at all.

I would run it just like you would for casting with sorcory dice being used for dispelling the effect instead of creating one. You would need a meterial link to the spell (As its the target) or the main caster (IMHO) or Eyes on target for it to work...

Just my take....

The group is the Hermetic order of the Auric Aurora.
They have jack in the way of material links, they don't know the target shaman and only know some bad juju is coming in the form of sea levels thanks to the PC's tipping one of their members' off. None of them have symbolic linking.
As such they only have the deductive work of a considerably knowledgeable mage.
Am less interested in raw and more interested in whats theoretically possible in the ways of magic, then get to the mechanics.

Well therory....

The might be able to see a build up of mana in an aera. Said aera might start to get a backgorund count as the spell formed. Spirits in the aera might be able to give more info on what is going on. As the mana is focused into the aera they might be able to trail is back astrally and track it to a source. Placing astral barrers in the aera might help to prevent the flow of mana fuling the spell. Astralling preceving the spell might allow someone with high magic theroy to create a counterspell that has oppisite effects to the one being cast...

Those are my thoghts to think over...

Interesting...
The Ritual of Doom tm, is a geographical effect. A recreation of the black tide in Britain/Netherlands on Downtown Seattle. As such it is a unique effect. There is at least one lodestone site created by a pre mini Ritual of Doom tm, thats part practice/part prologue to the main event.
As such the Order knows the toxic shaman is effecting the sea, and by having a bowl of sea water can effect a counter using the hermetic laws of Contagion and Sympathy.
In other words, they know the basics, its a geographical effect in an area they are in position too oppose. So thats the theory.
Devils in the details though.
As am setting a precedent I need to work out if the theory that yes they could, translates into, but the practicle details require them too.

Now the toxic is a lone magicain bug nuts enough to do this. He is off setting a BIG spell with a lot of sacraficed (murdered) magical life force.
The Order is, er, about thirty mages of differing levels getting together for some anti magic despell ritual. They dont know where or when but they know what and how. Which has any sane being fretting.

Commence brainstorm.
Sensing could perhapes pick up the mana spike of the ritual-? Or divining could help?
Astral projecting mages could scout Downtown looking for signs but that means less mages to oppose the badness.
Perhapes they could counter ritual before the Ritual of Doom tm, no that sucks.
Most ritual magic needs a link or eyes on to work, so they might not be able to do anything..
Astral watch on the sea could work....as thats the target...perhapes higher TN's or reduced pool to represent diffuse effort or linking difficulties...perhapes they could track the mana back to the ritual site...maybe they carn't all hang around indeffinatly waiting for this to happen...

Feel free to pitch in guys. Hopefully this explains what am looking at.

A couple of points here,
If the ritual group need a spotter to target the spell it becomes more a matter of an astral combat rather than a dispell. By virtue of the spotter from both ritual groups throwing down.
An interesting position is that as the Order know the sea is the target they could just watch the sea for the sending astral build up to counterspell.
But if they did it would make more sense to pool what would be close to 60 elementals and track the the ritual to source and kick hoop.

Finally, on p38 of MitS it says,
"Make a spell resistance test, if required. Allies of the target may provide Spell defence dice if they become aware of the sending before it is completed."

Could the Order just throw dice against the ritual of Doom tm? And if so would they need to see the sea to do so?

I will be going with a version of Tybus's ritual dispell, save the linking can be done to the target of the spell not to the spell itself. As the force of the spell is going over eighty I doubt this will do much good.

Dashifen's observation of working within the box is salient. As the mages are all capable magical practitioners they would make use of the most effective methods. As such a elemental hit squad is in the offing.
As a note, am extending sacrificing meta-magic in ritual magic, to be able to use multiple sacrifical beings. Inspired by a Azlan short story which featured the same and for over riding dramatic purposes.

I was uneasy of including DS so early in my creative choices, to those who did respond thanks.

ANytime and goodluck