Here follows a few tips on how to level up a character from scratch. The best way is always to be boosted by a friend while playing at high Torment levels. But if you don't have such an option, you can find useful these suggestions. Most of the following ideas are based on the fact you already have a well enstabilished lvl 70 character:1) Play at the higher difficulty you can. You must be able to kill mobs quickly enough, so use your judgment for this. If you're able to kill a Fallen One in 1-3 hits, for example, the difficulty is good. Remember that quantity is more important than quality: the more, the best. Remember that quests give more experience if completed at higher difficulty levels, so increase it just before completing one.
2) Items can have sockets starting from level 15. This means you can craft or search for socketed weapons and put into them an high level Ruby, in order to build a weapon with an incredible DPS for your level. This advantage will slowly lose effectiveness as you level up, even if you keep your weapon updated, as the benefits of the gem will slowly decrease. Ruby is quite good on helm, too, as it gives you a nice boost in experience gain. It can be worth to have both chest and legs socketed with Amethyst, too, in order to boost survivability and allow you to play more relaxed at higher Torment levels.
3) Rush levels, avoid single monsters and instead try to kill big groups. The bonus experience from massacre (kill streak) is incredibly useful, trust me. Because of this, try to use a character build that focus on Area Of Effect (AoE) damage and movement speed, changing it to a single target one for specific boss fights (like The Butcher). Why rush to the game's end? Because bounties are probably the best way to level up quickly, so the faster you unlock adventure mode, the faster you can do them.4) If you like to farm mobs, repeatedly do Act III most dense levels, like The Keep Depths, or Ruins of Corvus in Act V. In these levels you can easily build big chains that awards with huge amount of experience even at T1-T2.
5) When you reach level 40+, you can build a level 70 weapon with the "Reduced Level Requirement" quality. On a level 70 legendary, this affix range from 2 to 30 (falling usually around 13-18), allowing a character to wield such a mighty weapon much before it could, and giving him an awesome DPS. This affix is quite rare and elusive, but there is a trick to get it: craft a legendary weapon and make sure it comes with a "n% chance to xxx on hit" (where n is a number, and xxx a word: for exampe, 1,2% chance to Stun on hit) and another secondary stat (one among Life per Kill, Monster kills grants +X experience, Ignores Durability Loss or Maximum Mana, Fury etc.). Try to get a good one, with the right main stat for the character you're levelling up (like Strength for Barbarian) and Vitality or socket. Once you're done, run to the enchanter and just substitute the last secondary quality (leave the "n% chance to xxx on hit" as it is) until you get the desiderd "Reduced Level Equirement". You'll probably able to use the weapon once you reach level 50 or so, based on your luck, but it's still a very useful trick, especially for the last 10 levels, where craftable equipment is limited and hard to upgrade.
