The thing with Shell Smash in Random Battles is that it is usually didn't totally end your game before. Sure, you probably would have to sack a Mon or 2, but the thing was, it wasn't game ending. Now, with the everlasting joy in Dynamax, things just got a whole lot more difficult. If he sends out his Blastoise and forces a switch to set up a Shell Smash, he can immediately Dynamax afterwards. This results in a doubling of its HP stat, which now makes it a monster to take down, even with resistance to its common moveset, Hydro Pump and Ice Beam. I know from Experience that it's as hard as beck to take down those mons. Also, Status like Toxic isn't even really effective since they take 1/2 the damage from that.
Another probable factor is your team. This is RANDOM battles of course. As luck always goes, you happen to have half of your team weak to Water. This may have not been the case with you, but hopefully you see my point. Get stuck with a horrible team and you are done.