You could probably have it just exile two creature cards rather than having it be targetted as that way, they can't respond by removing one of the targetted cards to make it fizzle.

It wouldn't fizzle unless all the targets are illegal. But this is slightly mistemplated anyway, as you can't have targets in costs. The cost should be phrased like Everbark Shaman's ability's cost.

Also, beware: this is currently strictly better than Swamp. Needs to ETBT sometimes, or have some drawback to the mana ability.