Accumulating DECs is not so easy. Especially, when every second battle you lose. But I finally got enough DEC to buy 2 untamed packs.
I opened them as soon as I bought them. And these cards are not so bad. Well, I didn't get any legendary card, but I received epic and rare cards, which is not bad.
By the way, I am buying packs in https://monstermarket.io/packs, because it gives a cashback. Not so much, but still better than nothing.