I notice that no one has suggested hiding your link using a meta-planar quest
Many things bear an astral link to a magician that can be used to track the magician through astral space. Spirits, foci, spells, and material links are all things that can lead right back to a magician. It is possible to divert these links through the metaplanes, concealing the link to the magician to all but the most determined pursuers.
Now its up to your GM how he wants to run this, but I think this could be a very different session or two.
Another thing to remember is that he can only use the ritual link once: and if the attempt is aborted..the link is used up.
Defensive options:
Get your best spell caster to make a force 12 ward (painful, but that is why you have edge!). Sitting behind it you get +12 to all resistance rolls. If you have an active countermagic (say 6) as well that will stack..By this time you are getting +18 die on resisting effects. The ward will be useful to you anyway: it stops most of his magical scrying and sneaking around.
Remember that the spell caster has to be in a magical lodge of rating equal to or higher than the force of the spell. The availability of the materials is forcex2, so any thing greater than 10 or 12 will be unlikely. Remember that all casters need to resist drain, and its hard to get extra die resisting drain.
Having your best assenser is worth it too...It normally takes 4..6 hours to cast a ritual magic (twice as long if there are multiple traditions involved). You are probably only worried by spells of force 10 or more...so you assenser needs 8 to 10 hits in an extended test to spot it.This gives you options to boost your body/willpower with drugs, or be buffed by your friendly spell caster. The more powerful the spell, the easier it is to spot, and the more likely that you can astrally track back to where it came from
Most likely case: Force 10 damaging spell with 10 successes. You will be rolling edge on your 20+ resistance die, so will get around 6..8 sucesses. You are full of drugs, and have a medic on standby, you are quite likely to live. If the spell is much more than this, then he has the resource of a mega-corp (force 12+ lodges are hard to come by), and would probably have beaten you anyway.
Normally its quite grim for your bad guy: its quite likely that he (and his friends) will take serious physical drain. Sure he has a life pact, but he doesn't have infinite karma (otherwise he has already won). The damage you take from the spell is fixable magically. The damage they take isn't. An attack straight afterwards is well worth considering