So I throw four thunderstones...

[Malignor]

Does the DC increase by +2 per additional stone, or does the target make multiple low saves?

[Winterwalker]

There would be no stacking DC's involved here. Though four DC15 Fort saves are not always easy to make.

[Zerombr]

Sort of a shame that those alchemical items usage drops off when people can start making those saves very easily, and 1d4+1 damage is shrugged off.

[Winterwalker]

You could try pitting them, dip them in poison, then shoot them out of a rifle at people. :)