Squats, deadlifts and lunges are great leg exercises but if you want something more specific then try these. Get a resistance band and tie it around both ankles and then walk sideways for around 5-10 metres there and back while keeping tension on the band at all times. Or lie on your back with your legs in the air and hold a heavy medicine ball between your legs and squeeze it as hard as you can for 20-30 seconds. Put it down rest and then repeat. Both are great exercises for hitting the muscles used when holding someone in guard.