If I play the signature spell Death from below, and repeat it, does it stack to reduce the cost of battering ram?

No, playing Death from Below with the Repeat keyword does not count as playing two cards toward the cost reduction of Battering Ram.

According to Rule 820.3.a: "Regardless of the number of times a spell's instructions are executed with this keyword, the spell is only Played once."

Since Battering Ram checks the number of cards you have played that turn, and Repeat only causes the effect of a single played card to trigger twice (as specified in Rule 820.1.d.1 and the associated RiftJudge FAQ #9005), the game only recognizes this as one card played. Therefore, it does not provide the additional cost reduction for Battering Ram.