Mercies are a trash unit anyway. Extreme rng cancer because aa's are retarded. The ability to instakill full hp acus is insanely stupid too. With good micro you can survive 30 corsairs or 5 strats for some time, but you see 5 mercies come while ur aa is a bit behind or it randomly died and u cant do anything about it. Probably the worst unit design in the entire game.
Best idea in my mind to not make it bad is to make it have big aoe but little dmg and do damage over time. Groundfire infront of enemy army reinforcements to delay or dmg them. If you fire it in the middle of enemy army they can react and dodge damage because of damage over time so there is counterplay instead of "i hAVe aA". Also should have a human amount of hp and fuel.