Yeah to sum up, each class has an associated Primer and an associated Detonator.
Rogues:
Prime: Sleep
Detonate: Precision
Mages:
Prime: Freeze/paralyze
Detonate: Eldritch
Warriors:
Prime: Stun
Detonate: Impact
If you prime and detonate with the same "class" it only counts as a 'basic combo'. (I.E.: Wrath of Heaven + Shield bash, Freeze + Dispel, Sleeping power + Twin Fangs)
If you Prime and Detonate with Dissimilar class abilities you get a 'cross-class combo'. (I.E.: Pommel Strike + Mind blast = weakened combo, Sleeping powder + mighty blow = rupture, Winter's Grasp + Longshot = Shatter)
Some classes have two abilities that are cross class and those are the kinds that make it easy for you to solo the challenge.
Templar, Ele, AW, Avvar, Alchemist.