Yeah, if your minions are both immune, then it can go infinite in theory, and I believe that the attack cap on the Hysteria minion is likely above 15 attacks.

But I'm questioning if it will target another immune minion. Yes, your minions can target friendlies in such a case, but we have seen cases where "do a thing to a random minion" has had the effect where it will not target immune minions.

That is, if it is choosing a random minion to attack, it will never choose to attack an immune minion. The reason for this is that it makes a list of potential valid targets, then randomly picks one. But immune minions are not targets in that list making process. So this combo might just fall flat.

Now, somebody pointed out that friendly immune minions can be targeted, like Stealth can, but the question is whether or not the Hysteria attacks build the target list that way. I'm kinda thinking it might not.

Still, just Wretched Tiller + Deathspeaker + Hysteria would work without the second Deathspeaker as long as there is 15 total health in minions on the board, and it could be done for 7 mana in that case. Which is a small enough package to actually include as a win condition in some other type of deck.