Crits apply like you’d think. I’m not too sure how much base hp Hedge Knights have, but they can and do die with a fair amount of armor intact. I know for a fact that a crossbow trained brother with fearsome and berserk can get 2 kills in one turn on a good dice roll. A crit + kill on the first roll activates fearsome and the next shot can do heavy damage if it doesn’t outright kill the target.