From memory (admittedly unreliable), Sumia is supposed to be in one of the forward positions. She's only going to get directly attacked by one mage; the point of the maneuver is that the second mage will also move towards her to attack, which allows the barbs to get ahead of them. Mages will ruin the choke, since they hit too hard. The idea is to hold the line long enough to kill off the fliers first, before having to deal with ranged attackers.
But C5 is generally a Huge Goddamn Pain in the Ass<tm>. In retrospect, I mostly got through with luck. It might be less reliable than C2, something which would seem absurd in 2013 but is probably true in 2015.
Amen.