I have a Ravenbloom Student on the board and I haven’t played any spells this turn. What happens when I play Rebuttal targeting my opponent’s Stupefy and pay [A] on resolution? After everything resolves, how much will Battering Ram cost? Will my opponent’s Legion abilities be active?

Ravenbloom Student will trigger twice, Battering Ram will cost [4], and your opponent’s Legion abilities will be active.

Ravenbloom Student’s triggered ability checks whether a spell that you control resolves. You gained control of the Stupefy, and it resolved, so you will have triggered Ravenbloom Student’s ability. Battering Ram and Legion, on the other hand, check whether cards were finalized. Although you gained control of Stupefy, you did not finalize it; you got it after it was already finalized by your opponent. Thus, your opponent’s Legion abilities will be active, and Battering Ram will only receive a [1] Energy discount.