I don't know what you're talking about either.

When a fast moving fireball can do thousands of damage, there is a real risk in HC. I sent my SC Sorc back to try out damages - 7500 life with 75% resist all, and while she could tank most of the damage and heal (Moon thingies), she was
slightly threatened (not too bad, but in HC, you don't want even that low level of risk) if she took too many on. The Nova didn't do too much, nor did the Frost Orbs (although their damages could add up with other elements.) The Fire Elementals I didn't get a good look at, but at certain times, she was taking up to 2000-3000 damage in an instant. And with ~6000 life on the HC Railgun, that's a big chunk of life disappearing in a second. Get too many elemental attacks on you, and you can't dodge them all.
As to 95% resist all: Where are you getting +15% max resist all from? I don't think I've seen any item that gives % to All Max Resist - and even the odd one with + single elemental max resist haven't even been close to being useful comparatively. Unless you're referring to the Character Orb, which will have to be thrown away, so I haven't put much into it (mainly mana and some life). I think the sudden drop in max resists would hurt, late Terror/lvl120, and I may suddenly take too much damage due to overconfidence/carelessness.
Okey-dokey!
Act 1: I thought this was kind of cute.
A unique Porkchop! Nice to see some of the new monsters spawn as Unique as well, but not threatening.
Act 2: I got suddenly engulfed by a
hailstorm of fire. An amazing effect, but I never did manage to kill off these Heroic creatures. I got close enough to see what was casting it, but couldn't stand still or get the merc or summonables close enough to attack, while surviving the firestorm. They do have a useful drop, so I will have to go back and find these again.
Finally, the level 90 Challenge -
Tal Rasha
In this, you summon a bunch of Horodric Mages (or something like that) who give Immortality to Tal Rasha. Kill his mages, or lure Tal Rasha himself away far enough, and the Immortality drops. Problem is, the mages fire Nova (not that bad,) Cold Orbs (again, not bad, but the damages can add up if you try taking too many out at once, and it is much harder to dodge a barrage of spells.) The Fire Elementals target and can change direction, but again, aren't too bad. Normally they change direction into your summonables, so don't have to dodge too much. And finally, a damaging Fireball which can be dangerous.
Standard tactic of divide and conquer. Lure off a couple at a time, and slowly whittle down their health. Once they're in melee range of summonables, they're practically harmless.
Eventually, Tal Rasha was lured away, he began fighting in melee range, and there wasn't any further threat. A nice little challenge, but taking things slowly and cautiously, and the Railgun avoided most of the damage. (And apparently can be made even easier with items Railgun hasn't come across.)
LvlSix_Railgun has definitely exceeded my expectations, and her eventual death will hurt.
