After level 70 everything takes so long to grind, then there's exp penalties for dying... You'd probably spend 3 weeks trying to get a single level that might or might not make a difference, and fighting anywhere except the hardest difficulties and areas result in almost no exp.

Honestly it stops being fun if you are forced to grind that way.

I had a few amazons that I took to the mid-80s. Endless runs in only 2 locations in only 8-person games and it was still a hell of a grind. :)

I don't see why they crank the difficulty up that high. Yes it's suppose to be an honor to get to 99th level, but why limit it to 99? Why not 100? Why not 500? Why not a thousand? The options you're given are so limiting, and then later (since mid 40's) you wish you could get back your lower level skills because they are dead weight and are only used as prerequisites for higher skills. They weren't scaled to be relevant, yet you still have to waste the skill points on them.

If you want to make it all the way through the 3 difficulties and beat Baal on hell mode and don't want to rely on some fancy l337 build that requires fancy l337 gear and fancy l337 runes that never drop then I recommend a Teslafroster. This is a paladin build that attacks with zeal but you don't have to have more than 6 points in that skill. The only skills I maxed out were holy shock aura + lightning resistance aura (as synergy), holy frost aura + resistance against cold aura (as synergy). Your main attack is zeal which is on the left mouse button and on the secondary slot you can switch between the holy shock and holy frost auras. In the early game, I used only holy frost because in normal difficulty hardly anyone is resistant against cold attacks. In hell mode you'll encounter monsters that are resistant against both cold and lighting so when I see such a monster I switch to a weapon with high physical damage and use the might aura into which I only put one point (with "plus" skill items I raised that to 5). Zeal is a great basic attack because it only needs 2 mana per attack so under normal conditions I leech more than that back and I never have to drink any mana potions regardless of what attack I make. And zeal + offensive aura works with any crap weapon, of course you need a good one for hell and usable swords don't drop much, that's right. In fact, I only think phase blades are of any use. Personally I only used war sceptres. Found a decent one with Fool's mod on it (adds attack rating per character level, very useful) and two empty slots that I filled with Shael runes. For armor, I used a Dusk Shroud and put NEF + LUM into it to make the smoke rune word, those aren't high level runes and are easy to find in nightmare mode and it brought me all the way to the end of hell. For boots, gloves and belt I used Sigon's Steel, this is very simple level 6 gear, drops a lot. You only need those 3 things from the Sigon set for some really nice benefits, and it's easy to find this stuff. This is probably one of the least gear dependent builds for the paladin to make t all the way through. I also put a couple points on holy shield but not too many, only 4 (+4 points from gear) so 8 in the end. Before battles I activate holy shield and then switch to holy shock or holy frost and hammer away at the enemy. With enough points in Zeal to get 5 attacks per attack (6 points total is enough) plus a fast weapon you'll be hitting enemies so fast that you can hardly see the weapon, it's like Yoda smacking R2 with his little stick only 10 times faster.

EDIT: If you encounter a monster that's immune to cold, lightning and physical then run past it, not worth dealing with those freaks. None of the act bosses are like that though so don't worry.