Elemental minions?

[Luphey]

You can do something like Spherimorph from FFX, where when it takes damage from its elemental weakness, it switches its weakness to a random element. This would work decent on minions that you plan on making last for more than 1 or 2 hits. Flans are nice since they are pretty much already configured for any element, and there are plenty of sizes to fit your needs. Bombs aren't too bad depending on how high your ECL is for the party.