I just finished the campaign for the first time and I have one character at paragon lv 28, I don't even know what most of that stuff is. By "leveling up" I mean a way to level up relatively quickly as opposed to the 20-30 hours I spent on the campaign with my first character. Like speedrunning it on a certain diffuculty using a particular method.
New to the game? We got you covered. The easiest way by far would be to get powerlevelled by someone else, you might be able to get into a public game at Torment 6, tagging along for 4-6 rifts at most should get a fresh character to level 70 in this manner. You could also get into the official GAF clans, I'm responsible for the EU side of things, if that's what you want. Anyway, first thing you'll want to do is to start up adventure mode from the gameplay mode selection.
Difficulty wise, it's usually best to play something where you can 1-2 shot trash mobs and spend less than 10 seconds on champion and elite enemies. Efficiency is king.
The Gem of Ease mentioned earlier is a legendary gem which drops from Rift Guardians - end bosses - in Greater Rifts. To access a GR, you gotta run regular rifts from the Nephalem Obelisk in town, it's always by the blue glowing guy called Orek. Regular rifts will drop Greater Rift keys at the end, with which you can open a GR. Now the thing is that the first time you run a GR, you will always get the same legendary gem, Bane of the Powerful. After that, you get another random legendary gem each time you run a GR, until you complete the collection. Legendary gems don't drop after that, until you remove them from your characters inventory or stash or otherwise consume them with cube recipes.
With that said, you can't know when you'll get a Gem of Ease, with bad luck it might only drop as like the 20th gem. If you do get one, you can level it up via Greater Rifts. The formula goes gem level + 10 GR levels = 100% chance to upgrade. As you level a gem, you gotta try to keep the Greater Rift level higher than it to guarantee upgrades, because the chance diminishes the closer you get to the gem level. At level 25, the Gem of Ease will remove the level requirement of any weapon it's socketed into, so you could use a strong level 70 weapon on a fresh character.
The whole thing might be a moot point anyway, because unless you have a full set, you're not likely to be able to run GR 25, which is equivalent to Torment 6.
Regarding your idea of just trying efficient methods and routes, those do in fact exist. There's certain areas in the game which spawn big packs of comparably weak enemies, and in adventure mode you can warp to them at will. Act 1 has the Halls of Anguish, the areas close to the end of the act. Plenty of skeletons and some bigger guys. Ideally, you'd try to blow up a pack of skellies and hope to rush the next one to accumulate a massacre bonus. Those will multiply the experience gained, and at sufficiently high numbers, you can get multiple levels in one go. It can be difficult to keep the chain going though, since new chars have no movement speed bonus and little in terms of mobility skills.
If all else fails, you can always just run regular rifts. Reliable experience gain, constant gear upgrades so you don't fall behind and nothing to pay attention to but to kill mobs.
Anything else is just using what's available to you. Rubies in headpieces give bonus experience, so do certain craftable sets. You can extract legendary item powers via Kanai's Cube and use them on any character, which can also greatly speed things up. You ought to try doing the weekly challenge rifts! Those have you playing a fixed character against the clock, if you beat it in time you get a reward bag which also includes 8 of each act specific materials you need to extract item powers. On EU, the challenge resets every monday at shortly before 11 PM GMT in the evening, so you could still get the bag of the previous week and another again really quickly. It's useful because running bounties is the only other way to get act materials, and that's tedious.
. Chapter IV is indeed the last chapter per se, but there's 5 more tiers after that. The 5th tier is just for show and cosmetic rewardss, but the 4th tier is required if you want an additional stash tab.
