If you did go the Mystic Adept route, there's nothing specifically saying you have to allocate points to spellcasting to use Counterspelling.
If you have Runner's Companion, the metagenic quality Arcane Arrester could be nice. Either Surge III or take the metavarient race Fomoroi.
QUOTE
Arcane Arrester
Cost: 25 BP
When affected by a spell (including a critter’s Innate Spells),
the character—and she alone—treats Force-based effects (damage,
paralysis, etc.) at half (round down) actual strength. Note that the
actual Force of the spell is not actually reduced. For instance, a
character with Arcane Arrester targeted by a Force 5 spell would
resist it as if it were a Force 2 spell, though the spellcaster could
still add hits to improve the effect. Arcane Arrester cannot be
combined with Magic Resistance (p. 79, SR4). This quality can
be taken by characters with a Magic attribute.
If you were to change the concept into a mundane with a ton of cyber/bioware, another quality from Runner's Companion could also be quite devastating as a mage slayer(and LOL it's a Negative quality):
QUOTE
Astral hazing
Bonus: 10 BP (for non-Awakened characters),
15 BP (for Awakened characters)
A character with this quality has expressed metagenes that
somehow catalyse and feed on the character’s darker emotions and
negative feelings, disturbing the character’s aura and any ambient
mana in her vicinity. For reasons not yet understood, the character
becomes an aspected domain in her own right and taints astral
space around her wherever she goes; a generator of tainted astral
background count (similar to a cyberzombie).
This astral haze afects all attempts to cast magic on, at, or in
the vicinity of the character. Whatever the ambient mana condi-
tions are, the character always stands at the heart of a Rating 4
background count (see p. 117, Street Magic) that extends a num-
ber of meters from her body equal to her Essence; this background
count also impairs her own actions if the character is Awakened. If
she remains in one place for long, the background count expands
by one meter in every direction every two or three hours (at the
gamemaster’s discretion).
Hell, you could be a chromed up Fomori with Astral Hazing and an essence of 1 and really go to town on mages. Of course, that concept is quite counter to your fluff ideas.