Instead of pushing the board under the wave, you roll yourself and the board around its long axis and let the wave pass over both of you. The answer for boards with too much volume to duck dive.
The duck dive fails on guns and longboards for a plain physical reason: you cannot push that much buoyancy under water, so the wave hits you and the board side-on with full force. In a turtle roll you grip both rails at around hip height, stay beside the board rather than lying on it, and just before impact you roll the whole package around its long axis — deck down, you hanging underneath. The decisive error is an air gap under the nose: that's what lets the wave tear the board out of your hands. Lay the nose flat on the water and hold steady instead of bracing against it; the pressure is usually through in a second or two. What often gets missed: after that, every second counts. Roll back and start paddling immediately — on big days a wave rarely comes alone.