A counter spell is usually about 1/2 magnitude of the attacking spell, to negate the attack, and as long as the competing forms make sense. CrAq against CrIg. PeFo and PeVi are always good fallback mechanisms. The Dimicatio is a pain to compete in, one almost has to be hyper-optimized for it, which makes him a combat monkey with little else going for him. See Marcus of Tytalus for an idea of what I built to do well in the Dimicatio.