I've been playing an unliving deck quite a bit recently, and I find this card kind of "meh". I don't want to spend one of my own rares to get a rare from my opponent's deck. I feel like that's basically all this card is, other than the other minor costs (an extra turn, 10b10g) and the other minor benefits (+15 wall, 14 or 21 damage).
The only place I could see it being useful is in a rush deck, since it's one of the few cheap rares with simple offensive/defensive properties. Overall though, I really feel it could use a buff.
When used in an unliving or illusion deck, there's a good chance you'll already have a mirror golem in hand (making its replacing effect redundant). Maybe a buff could be to give it some kind of amplified effect if that's the case? Or maybe a conditional based on "if #unliving > 1 and #illusion > 1...".