I'm loving the whip down. Hitting a leg lock on anyone at Empire has grown considerably more difficult over the years and that one from MTS 15 is no exception, but because everybody's afraid of the ankle lock you can use the whip down to get them to shift their weight and switch to the whip up. Gotta have your shrimping together for that, but it works.
Core work will help of course, but it's as much technique (or more) as anything; even with the heavy guys. STRETCH that lockdown before you begin to whip. That moves his hips further down and takes some of the pressure off of your upper body; especially if he's a big guy, you aren't whipping him anywhere if his hips are higher than yours. Coach always points out this detail too: try to knee him square in the ass when you're whipping. Extend the lockdown, heels up, knee him HARD in the ass, and shrimp to your side. If your lockdown is solid you won't really hit him in the ass; the mechanics of trying to do that with his leg locked down will lift his hips and create the space you need to get to your side.