This is a very advanced strategy, and since I don't actually have a group to do the full testing with, I'm going off some simulated data.
This requires 4 players, 1 of them being a necro. The other 3 don't really matter.
The group is designed as follows:
The necromancer (The leveler)
The cursed chest finder (class doesn't matter)
2x blood goblin farmers (class doesn't matter)
Vendors to check:
Act 1 Ring Vendor (teleport to Weeping Hollow for quick Access)
Act 3 Ring Vendor (Bottom Right)
Act 5 Ring Vendor (Bottom)
Necromancer skills (equip when available, otherwise, use whatever you're after)
Corpse Explosion: Close Quarters
Blood Rush: Potency
Command Golem: Flesh Golem
Bone Armor: Vengeful Armaments
Bone Spikes: Sudden Impact
Land of the Dead: Frozen Lands
Commander of the Risen Dead
Run the challenge cache.
Unless they changed something, as long as you complete the challenge rift after the season starts, you can start ahead of the season. Last season, this didn't work as the season didn't start if you were in the challenge rift, so it may have been corrected. I'll be checking on Asia though when the new season starts.
Start the game with one player who will buy and craft gear for the necromancer (the chest finder). The other two people are searching for blood goblins. When they find a blood goblin, hold it for the necromancer..
Gear to buy:
Act 1: Weapon (preferably with +XP) and the 4 armor pieces.
Gear to craft
Level 5 Axe
Level 5 Phylactery
Level 12 Sword
Then, the non necromancer will exit the game, and search for the 3 following Cursed Chests on T6 (make sure to stay Level 1. You're not to level up at all):
Cursed Spire (best)
Cursed Bellows (worst)
You'll have more than enough time to find one of these. Once you find it, just hold the game (stay at level 1!!!)
The necromancer will level up to 22+ (grab the cube or whatever is fastest). When 22+, make sure to reform a game to buy the best vendor gear you can. Total time: ~15 minutes
Hopefully a blood goblin has been found by now, and the necro will have some shards roll Grasps of Essence. The necromancer will cube them, and select them. Then they will join in the game of the chest finder. The chest finder waits at the waypoint. Run the cursed chest. If Level 34+ after this cursed chest, skip the next paragraph.
If the necromancer is below level 34 at this point, both players exit the game. The chest finder will search for one of the following chests
Cursed Cellar (do not do on hardcore)
The necromancer will enter their own game and buy the best possible gear again (switching skills when necessary, etc.) They'll join the chest finders game and repeat the process. The chest finder waits at the waypoint, and the necromancer will run the cursed chest.
After the cursed chest, the necromancer will be level 34+, and the other player will be level 28+ (depending on the chests found). Total time to get to this point is 20-24 minutes.
Now, the necromancer will hopefully have enough blood shards to roll Custerian Wristguards. If they get them, equip them, and then open up and collect the challenge cache rewards. The gold will boost them to level 54. If not, oh well.
The 3 other players will now run Acts 2, 3, and 5 bounties. It will probably take the necromancer 15 minutes to hit level 70 from this point onwards running cursed chests (Look at the above 6 chests). Make sure to craft a Level Reduce Weapon with at least Life on Hit (required), and preferably Vitality.
With the above steps though, softcore was a similar time, and hardcore would have shaved off 6 minutes.
Without Custerians (depending on the level reduce weapon):
Softcore: ~40-50 minutes to hit 70
Hardcore: ~55-65 minutes to hit 70
With Custerians (depending on the level reduce weapon):
Softcore: ~35-45 minutes to hit 70.
Hardcore: ~40-50 minutes to hit 70.
Once at 70, the necromancer will do the boss kill portions of the bounties, then collect the bounties from their teammates. Runs Acts 1 & 4 bounty sets for the jewelry recipes. Remember that you can purchase 3 jewelry patterns from the Act 2 vendor. The total time to complete the first 4 chapters should be roughly 30 minutes because of your teammates. Then, just power level them to 70. It takes roughly 6-7 T6 bounties to go from level 48-70, assuming they wait in the zone to collect XP. Don't just go to rifts though, get their chapter requirements out of the way for the leveling. At about the 1h45m mark, your entire group should have completed the first 4 chapters and everyone should have their 6 piece. Then, just continue to level up however you want