HA! Don't let them get set and get their hands/arms in position, no matter what guard or other technique you're using. The Mechanic sweep is a good option. Don't just settle in either. Keep attacking them. If you keep aggressively trying to sweep/attack them, they will be more concerned with defending. Also remember, the hardest thing for a larger opponent is to keep up with a smaller, faster opponent. You can't stay still too long with a bigger/stronger opponent. That's what they want. If you can get to Boa, attack them with sweeps or trying to get to their back immediately and often. Good luck.