Okay, I think this is deserved for a number of reasons. It is powerful, versatile and decently cheap. I feel this is a lot for a card. Also this is often the most frustrating card in the dreaded Firestall, of which I feel will weaken a bit if this nerf is passed. Here is what I propose:
It basically loses lethality, but it makes the creature as fragile, yet powerful, as you would see a typical Fire creature. It still has potential in Firestall, as it can be synergized with Fire Shield | Fire Buckler for the final blow, but it requires an attack to work, making it a little risky.
Just in case wording is a little confusing, some examples:
Sapphire Charger (4|5) with normal effects of Rage Potion = 9|0 (killed)
Sapphire Charger (4|5) with suggested effects of Rage Potion = 8|1
Colossal Dragon (7|15) with normal effects of Rage Potion = 12|10
Colossal Dragon (7|15) with suggested effects of Rage Potion = 12|10 (same potential)