HCG is not "completely necessary". However, it is highly recommended.
Aromasin is a good choice for an AI as well. It is often more expensive than Arimidex however.
What happens without it? Your balls just shrink? If that's the worst that can happen, can't a person just accept that during on cycle?
I was thinking something like epistane which has the suicide inhibitor and prevents rebound and doesn't need to be tapered off.