Partially based on: http://www.d20pfsrd.com/magic/variant-magic-rules/spell-duels
Rebalancing might be needed at a later point
Preparing to counterspell is a normal action, which uses 2 actions.
If the target of your counterspell tries to cast a spell, make a Spellcraft check (DC 15 + the spell’s level).
Note that characters who cast spells spontaneously (such as bards and sorcerers) must choose what exact spell they are using to counterspell in addition to the slot being used. The countering duelist must then make a caster level check against a DC of 15 + the spell’s caster level + “other modifiers”. The caster attempting the counterspell receives a bonus or penalty on her check depending upon the level of the spell slot expended and the exact spell used, as noted in Table: Dueling Counter Modifiers. If the check is successful, the spell is countered—it is negated and the spell is lost. If not, the spell happens as normal and the duelist attempting to counter the spell takes a –2 penalty on any saving throws made against the spell’s effect.
+5 for the first assisting caster, +3 for the following, +1 for the rest (the larger the group the greater the power, but the more minds the more variables) The greater the power the larger risk.
^ remains the same (I think)