You have several different points, so let's tackle them one-by-one.
1) Zealots do a lot of fire damage in an area-of-effect attack when they die. It's takes a few tries, but you should be able to get good at dodging that attack. It only happens when they die. So as you strike the killing blow, get ready to run back a bit. The damage also hurts other zealots so it's possible to set up a domino effect where killing one zealot ends up blowing up a few other zealots, who in turn blow up. Your Companion won't dodge that attack so he or she will get caught in the blast a lot.
2) There are a couple of places you can get an anti-rad helmets before the Arc. They're tucked away pretty well. See
here for details. The Wiki has a lot of useful information so it you haven't read over it before, give it a glance or two. Even with anti-rad protection, the red orbs will some damage, but it will be much more bearable.
3) The Spirit Cleaver is a decent all-purpose weapon. However, you're not getting the most out of that weapon against minotaurs. The Greataxe of Falling Stars, or Soul Drinker will preform a little better. On the greatsword side, Rancor and The Scimitar of the Wolf do a little better, too. The Wiki can give you more details.
4)
Bappasalar Cave, the cave on Southern Dragon Mountains, hits hard with fire and ice damage coupled with a few dragons. It can be tough. But it contains a few good items. A strange hood, access to make a Adamantine swords, so decent high loot. It will likely take a few trips. Goal is to explore it bit-by-bit. That will give you plenty of time to learn how to reliably dodge exploding zealots. With a little effort, you can lure the dragons to fight them one-on-one. You can try the run-past-them-and-warp-out approach to the guarded chest. Between your companion, a summons, and a speed potion, you -should- be able to get to the chest, loot it, and than use a Scroll of Recall before the dragons kill you.
5) Once you get a strange-hood, getting to the Sunken Citadel will be a lot easier. You'll find the the monsters there drop some pretty nice potions, better gear and a hefty amount of XP. There's a puzzle to solve, but if you get stuck, there's the Wiki. Also, once you take out the boss in the Sunken Citadel, you'll be able to warp between the Citadel and a City, making repeat trips a lot easier.
Good luck!