Missile Guide is a three-stage chargeable skill. If you got hit by it for 6k before, it most probably was the first stage and NON CRIT.
The third stage of the skill does roughly 67% more damage than the first, and a cannon's crit multiplier is x1.5, so the calc is 6000 * 1.67 * 1.5 = 15030
See you ingame.