Play some more premium hands.
If they are often seeing the flop then they have a super wide range of cards and can pretty much have anything on the board each time.
This means they could be playing a lot of A2-3s, K8, 4-7, etc.
Playing premium hands will allow you to take advantage of this since If you see the flop with say Ace-Jack and you hit the board, but they have an ace-X card as well. You will likely have the stronger Ace and win a good amount of chips from betting since he isn’t folding that ace but doesn’t realize an A3 is relatively weak and will lose to just about every other A-X card.
Of course if they call to the river then shove there’s the possibility that they just hit a mister hand and want all your chips now. I’ve called enough times to see this as an often scenario.
Them checking often postflop will also likely tell you when they don’t have anything or they would be betting because they’ll think they have the best hand with only a pair
Just some experience I’ve had against these players