I don't know guys, I really like the 2.0 difficulty changes (apart from the 'no-rune-downgrading thing' ofc

). Finally you can't storm naked through everything that is not an uber (well, some builds probably still can), you have to actually play the game. And that is a good thing. Actually, in my opinion acts 2-5 need more tinkering, those feel way easier than A1. That's because in A1 you don't yet have merc, resists, core skills, actual weapon... and the game don't take that into account.
@OP, I don't know your in-game experience, but maybe those few tips will be helpful:
- hire a mercenary, that's a MASSIVE help early on. If you are ranged, take fighter mage (A2), if you are melee take necrolyte (A3). Those have GREAT crowd control abilities and will tank VERY well, even without gear. At higher difficulties you should give them at least some resists and defence though, also easy to find/craft items with CC procs, like Toorc's Gift, Sea Summoner or Bogspitter.
- stack resists. As you noticed, there is abundance of elemental dmg in the game. Pick up every bloodstone, onyx, amber and turquoise you find and upgrade them to perfect versions until you have like 6 of each. Socket them in your gear. You can also use mystic orbs with resists on jewelry, preferably rings. You probably rarely change them anyways, and big improvement on those is even rarer, so it's a safe slot for some patch-up.
- although there are way more ranged and elemental threats in 2.0, many of them are telegraphed and can be completely avoided. Yes, simple stat checks are less frequent. Don't believe me? Just compare Arcane Sanctuary from 1.8 and 2.0. Try to squeeze every movement speed you can, you want it more than ever. Dodge what you can, hide behind merc, corners, and obstacles. Use sprint button, use blinks (easily accesible from Pax Mystica runeword). And remember as a rule of hand, the slower the attack, the more it will hurt.
- try to reach decent attack/cast speed and hit recovery. The faster your animations, the faster you can perform other things, like moving. Or just kiling things even faster

- as for Diablo fight, you need to kite him all the time. Try to make him focus on yur merc, or since you have party of two you can play fool with him. Don't stand still in flamestrike or lightning. You can potion-tank fire nova or, even better, blink over it. Make your portals away from the fight, maybe near the seals, so you won't die by entering them. Don't be greedy for a quick kill, better to make it longer and not die. Max your fire and lightning resist. If you want to be extra safe, make a shield with Stone runeword. "Cannot be frozen" is a wonderful affix and it will save you on many occasions, not just this fight.