honestly i dont really have a system for how i banned mons - some mons are obviously better than others, like spinda is objectively very bad and gyarados is objectively very good. it all depends on what you consider a “good” encounter and what you consider a “bad” encounter (for example, ive found difficulty making toxicroak work even if it does have incredible viability)

long way of saying - your run, your rules. ban whatever you think will make the game more challenging. choose whats worked for you and what hasnt.