On the SBOG section in Vietnam, I can't get past the first part of the trench. A VC automatically jumps on the player and the screen says to hit the square button as often as you can. Have done that and also tried the X button repeatedly but player remains on the ground as it fades to white for another retry. Have tried to hit square button and x button and plant grenade as mentioned somehwere but player still remains on the ground fading to white. Can someone advise how to get past this?