The biggest mistake people make when it comes to leg raises is initiating the movement with their hips. Big guys, with their long, heavy legs, usually make it worse since the hip junction is the only pivot point. To properly hit the abs, focus on rounding the spine while bending the knees.

If you struggle with leg raises from a dead arm position, or the overall loading is just too much to handle, there's no shame in forfeiting some of that load by resting the upper arms in sleeves.

This allows you to focus more on trunk flexion and having eccentric control through the negative half of the lift.

Related:  The Oblique Builder

Related:  Core Training for Big, Heavy Dudes