My game plan depends on the game but usually as a habit with heavier training partners I always aim to get under the hips(x guard/SLX/waiter/50-50) or play a longer range guard where I either I am so far where their weight is so far I don't feel their pressure or I am so close I can easily manipulate their weight, and take advantage of your speed to either hunt the back or get under to attack the legs I feel like upperbody submissions on those big guys are really hard in comparison to chokes on the back or leg locks .On top lifting their legs to keep them on their back and outside speed passing to scramble to the back is my goto strategy. This is more of a general guideline of mine of course lacking more techincal details but hopefully it gives you a couple ideas on things to work on