With rentals becoming more common, we see Llamas ever so frequently now. What used to be a rare sighting is now at least once a day for me. Paired with a Kron, Flesh Golem or other self healing card, it can be quite hard to beat.
That said, it isn't invincible. If your last stand monster is a weak one that can't self heal, it won't make much of a difference against decent opposition firepower. In the title screenshot, my opponent lost the battle because his last card was weak. Very weak.
Here are some other tactics to beat it in the lower leagues:
Last Stand is a new ability out of the Untamed series and one that takes some getting used to both to use it and as an opponent.