In my experience with a great many fishymancers is the difficulty curve goes in reverse.
Normal can be a total pain in the ass quite frankly. Diablo can be particularly bad. His fire nova can 1 shot your whole army. However as the leves pass and you start getting more points into the summon tree and particularly get your summon resist to around 50% (or stop at 17 points after gear) you can just walk over anything.
Go and revive a load of the mobs from the durance of hate that have crushing blow. Bosses die easy then.
I should point out I've only ever played Diablo 2 online twice. My whole 8 year on and off career has been single player, self found, usually untwinked, mod free play.