Hardest boss in all of gaming?

Asmon is quite bad with multiple enemies (a self-reported issue), so even if these toads are on the harder side, for him they must've been abnormally difficult.

Also, literally the simplest but huge increase in difficulty would come from a requirement to kill each of them within a short period of time of each other. I think some dark souls had that mechanic and even L2 (in a later update) had a twin boss that you had to kill within a short time window.