You can choose not to put out a card when using genesis wave. It will be a wasted card, but it does not have to kill a legendary that is already out. Just choose to put that card in the graveyard instead. I have genesis wave in a similar elf deck, and that is how I use it. Hope that helps
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Elvish promenade is not making druids, and this deck is built to have elf druids in order to use gilt-leaf archdruid. Since it is creature-based I did not include beastmaster's ascension although I had it in there originally. Priest of titania is a good option to replace the two arbor elves maybe, but mana is definitely not a problem in this deck.