I hope I'm not missing anything obvious here, but it sounds like you need to keep track of how much time has passed before shooting, so every tick, each enemy will need to accumulate the time to know exactly how much time has passed since the last time they fired. If the accumulated time passes, that's enemy's fire rate, fire off a shot and reset the accumulation. Therefore enemy 1 can have a fire rate of once per second, while enemy 2 can have a fire rate of n per second.

I hope this helps.