If nothing works on them, and there is only one option left:
Throw them into the sea, like Garp did it vs Sanjuan
But Saturn somehow managed to maneuver himself mid-air and turned back, so this might not be an easy thing to do.
On the other hand, this showed us one thing: Saturn wasn't really careful with blocking other attacks since nothing damages him, once Luffy tried to throw him into sea, Saturn suddenly reacted and didn't let that happen, while Luffy's attack wasn't successful, it shows that falling into the sea might be Saturn's and other five Elders only known weakness so far.
Perhaps this is why Darkness and Quake DFs were very important for Teach, if he knew about ''immortality'' as its implied at the end of Law battle:
We know Doflamingo knows the secrets of Celestials as well, and also knows about Immortality and National treasure, if Teach somehow learned that too, perhaps the only way to beat them either throw them to the sea or cut their energy source.
The name of Teach's attack:
''Darkness Water'', it imitates the Sea Energy so that the DF user can't use their power similar to falling into sea.
Why Quake DF was important?
Large scale attacks can cover the island with giant sea waves, can send Tsunami to even an island thats far away, if they are covered while Tsunami sinking the island, then there is no escape from the sea.
That ability is also a big advantage in the sea battle,
Teach didn't even let Law escape underwater, he can control the sea with Quake DF,
''king of the seas'', thats what Teach wants to be.