The solution has been suggested over and over - they need to add a clause which means only full lines are selected for expo and crypt. Unfortunately I don't think Gree has ever responded to this - it doesn't seem to be on their radar.
I've generally been lucky when it's happened to me and it's been a physical damage hero so I just solo spectral to kill it. If it's tasty Spider can be useful. I'm afraid that's the only advice I can offer, except to suggest that when lowering defences for contests people use a full line of heroes, or a single white hero.