Most of the senior Xbox online hunters are Sorcerers and/or MKs, and I'm one of them. If I take a kill on this gen, it will be my 60th; I built my main GT out of ignorance as I didn't find GF until late August and I was already far past the point of any strategic leveling. So there's all the boring background.
I play a 1/4 Sorc, 3/4 mostly MK build. I have not have any trouble whatsoever taking online kills as an MK; I have 1 as Ranger, 30 as Assassin and the latest 6 are MK. The biggest hurdle to using the vocation that suited my build best was being told repeatedly that it wasn't feasible because my stats weren't strong enough. I find that to get the best damage from your MK, range counts-- firing a GC from a distance means most of your power is eaten by that distance. I fight directly under the Ur's feet, so my higher def from the MK's leveling pays off as I am not particularly phased by incoming physical damage.
I took my first MK kill in two rounds. The rest have been single-round kills. When I want efficiency in taking down the online Ur I can always bring out my min/max assassin alt, but for pure unpredictable excitement and high level violence it's MK all the way, baby.
More to the point, damage testing with a stripped down MK proved that the magic cannon stacks both str and mag into its bolts. I am not sure where the prevalent idea that only and ONLY a magic-only build would be acceptable came from; my main GT's max damage in one blow/foe is 2.4 million, acquired during a round against the online Ur.