you are right, have no doubt,the game will calculate and analyze what deck you are trying to make, and will try to do everything to make you not get it, this is not a joke
Trying to build one as well, funny thing is I'm down to 94 packs left, and I still haven't got a Murmillo. lol. Also still have to get the second Laquari. Was lucky enough that the first one and the 'impenetrable attack' were in the top part of the pack.
Pro hint: Never go to the shop right after entering Deck Editor. Game sees the deck you're working and will give the cards needed in it only in the end of a Box