Although zellbrigen denies any interference between the single combat so artillery is not the prefered choice, but Clans DO use artilleries, and against dezgra - and chalcas - zellbrigen is not even a consideration.
-----------------
Also, remember the duel between protomech/battle armor and battlemech, and modified zellbrigen of aerospace fighters. Aerospace fighters aren't follow the code of duel when they against the warship, and a dropship can engage a squadron of aerospace fighter rather than only one fighter. Also a point of protomech/battle armor can issue challenge against each other point of them or the other battlemech(that is basically a point) as if the whole point is just a single entity.
So, it is possible that, even if they follow the rules of zellbrigen against the superheavies, it is possible that a whole mech star can issue challenge, NOT a single mech, and any number of protomech and battle armor can engage the superheavy just as any number of aerospace fighter engage a warship.