Lone3wolf: The more people in the party, the stronger the foes get, so yeah, it's normal for MP as well.
Gaunathor: I still find it weird, that 8-players-mode in SP is easier for me, than 2 players in MP.
Lone3wolf: We probably are somewhat under-equipped (I know I, and my Bitch rogue are to say the least), for some fights, but the right skills-use gets us through them, mostly in one piece.... >.<
Gaunathor: That's most likely it. My resistances during the fight against Diablo were pretty low. They are better now (maxed Lightning and Cold, Fire is pretty high, only Poison is still low), but I'm not sure, if they are good enough for Nightmare. I'm probably going to farm for better equipment first, before I switch to the next difficulty.
Starting a Nightmare run (next difficulty) will get us access to mostly better equipment, but your resistances are lowered by....20%? something like, anyway, before you even add equipment!
The trouble is finding runes and gems for the socket-able stuffs, which is where resistances and buffs come in. rings and ammies do a little, but you're gonna be really lucky to find anything worthwhile.
Might be worth going through normal acts 1-5 bosses a few times and see what drops off of them, though.