They can play up her toughness and grit all they want. Castle is a pretty big guy (they mostly hide it through camera angles and not letting him do physical stuff, but look at some group scenes of several people in a hallway or tight space and see how he looms over all the other guys on the show), and Beckett is a fairly skinny woman. If a guy who is capable of tackling Lockwood to the ground and pounding on him can't pick up a chick who weighs a buck-20 tops when her life depends on it, you've got serious plausibility issues. The real suspension of disbelief for that scene comes with the whole notion of getting her and Castle out in the first place. It was done solely to give Montgomery a heroic death, when with Castle and Beckett hiding in backup, he probably survives. As it was he took down all his killers alone. The way the fight played out, there was hardly a sense of inevitability about the outcome.