Welcome to Gaia! ::

I am now back to playing my Crystal Scraps deck for this format, for fun more than anything. (My other decks are Resonators (un-finished) and Scraps (waiting for Orthoros)) But i have a question about a tech of mine.
If i have a faceup Ancient City Rainbow Ruins on the field and at least 1 crystal beast monster in my backrow as a spell can I use Scrap Dragon's effect to select Rainbow Ruins and my opponents card to get a free destruction? As in my rainbow ruins wouldn't get destroyed? Or do you have to select a card that can be destroyed?

■If one of this card’s targets for destruction is no longer on the field when this effect resolves or cannot be destroyed when this effect resolves, the other card is still destroyed

This would mean that Rainbow Ruins would be infinte fodder for Scrap Dragon?
I only ask to double check because i played this tech today and my opponent said i was full of crap. So i asked someone else and they said i had to choose a card that could be destroyed.
I was like: Eeeer no, it isn't a cost therfore i can choose it and it won't get destroyed.
I sent that link to the guy i faced and he said: Well if it is the case then it'll get sorted because that's an unfair advantage.

wtf!!!! Please tell me i'm not crazy! Aaaah