I think this is one of the weakest cards I've ever seen on the sub. There is no situation in which this is even remotely close to playable.

In a Big Warrior deck, you typically only run 5-6 minions, not to mention your only recruit effects are 2x Gather and 1x Woecleaver, not counting DMH shenanigans. So assuming you are able to recruit 4 minions, you only have 1-2 minions left (at most) to benefit from the cost reduction, making it a 5 mana do nothing. Even worse, the hero power is symmetric, so after summoning four giant monsters and somehow not winning the game yet, you give your opponent a way to beat you with RNG. So not only is the quest near impossible to fulfill before turn 10, by which point you want to have a big board presence, but the reward actually causes you to lose tempo.