I am a level 20 warrior that never used magic with normal plate armor and Griswalds is not selling any premium armor.
You're generally better off going with Arkaine's Valor unless you can find something
really good. Faster hit recovery allows you to keep swinging even if you get hit. You'll blow through potions, but you'll win the fight.
As for magic, there are three really great spells for the warrior. They shouldn't be necessary, but they're really nice. The first is firewall, which is a practical way to corral fleeing monsters. Stone curse is by far your most dangerous spell, turning powerful enemies to rubble when paired with a few swift attacks. If you can get somehow get a massive boost to your magic score, teleport is absolutely awesome for a warrior, but that's hard to hit the prerequisites.
I think your problem is that you are playing on Hell
I suspect he's not on Hell difficulty, but rather referring to the level 13-16 dungeon tileset. It's a common beginner mistake. Even ignoring the difficulty of attempting hell difficulty directly, the amount of XP given out in hell difficulty means he'd be well above level 20 before even finishing the catacombs.
No. The boss before him is a lot harder.
Disagree; so long as you don't stand still and get nuked by the bajillion spellcaster enemies standing nearby (
take cover) the penultimate boss is actually incredibly fragile and easily slain. Just don't stand in the open, the fight's not that hard.
The key difference with Diablo is that he can do things that previous fights cannot. He has a ranged attack that can hit you from well over a screen length away that's effectively impossible to dodge, he's immune to any spell effect that isn't damage, and he's not dependent on his minions for his difficulty. This means that players who utilize range, cover, and stealth to pick apart stronger groups of enemies will find their normal strategies useless against Diablo, making him
by far the most deadly enemy they will encounter. On the other hand, people who grind for superior gear and levels to power through rooms full of Diablo's elite minions will find that big D himself isn't scary.
I tend to tackle Diablo around level 21-23.