Killing normal Diablo
If it doesn't have an effect in melee range, I wasn't aware of it. That would be info for me. It seems to fry my merc and skellies in melee range something awful.
I've read that you have to be at melee range 1 in order to enjoy some protection from the lightning hose, and while that may apply to the skeletons (who take 500% damage from act bosses?), the merc is probably at melee range 2-5 depending on what his weapon would allow for. Not that I know for sure that standing really close lessens the damage.
But there's no real need to worry, because I've found out that any necro with enough dexterity to get max block can
tank normal Diablo himself, at least at level 34 or so. All it requires is a weapon with decent damage, such as a Vicious Blade of Slaughter (Damage: 5-41), and maxed resistances to fire and lightning. It's slow going without Crushing Blow, but with Life Tap the necromancer can easily leech back life faster than Diablo can take away. The clay golem was at level 9 after +4 from items, and the necromancer was wearing Hawkmail, but not using any cold charms of his own. So I think it evens out in the end. Strength 76 might be higher than any fishymancer would have at that point, but daggers get half their damage bonus from Dexterity, so it should still work with a lot lower Strength.
I was originally going to see if the Inner Sight from the rogue mercenary would make it much easier to hit Diablo, since she'd take his 200 defense down to 0. But she must've done something wrong, because he ran straight for her and hosed her down. So I decided to have a go without that kind of assistance. Apparently a 65% chance to hit was enough, and what little damage he did was quickly leeched back. And thankfully he completely ignored the clay golem, so I didn't have to worry about summoning that one over and over.
I suppose that using a Strength weapon or any of the normal uniques with Crushing Blow would work as well, but the damage may be too low to work well with Life Tap, so maybe then Decrepify would be a better option. Or maybe it's just not worth the hassle, and retreating to resummon the skeletons would be preferable to changing tactics for this one battle?