Blizzard Watch have posted another guide to Demon Invasions, including how to quickly level alts and/or gain gold on neglected realms:
Invasions are a great way to gear up your level 100 characters to face the challenges Legion will hold. (Because even though you’ll replace gear pretty quickly in Legion’s starting zones, better gear will certainly let you have an easier time until you get it.) You get level 700 gear drops (that can be warforged to 720, with weapons that can be upgraded to 725) which will be a heck of a boost to players who haven’t been hitting Hellfire Citadel. You get cool, Legion-appropriate transmogs. You can grab a pet, more transmogs, and more gear with the Nethershard currency that drops from demons.
But invasions also scale to level. Any player above level 10 can jump into an invasion zone and have a fair shot at fighting demons. You’ll get level-appropriate XP for everything you tag when it dies (so tag away!) and big chunks of XP whenever the invasion ticks over from one phase to the next (as well as chest with loot at the end of phase two and phase four). In the level ten range, you can get five or six levels from a single invasion. In the level 90 range, you might get a level every two invasions. And that experience stacks with XP buffs from heirlooms and more, which makes this the best time to dust off those alts and get leveling.

Demon invasion basics
First, let’s review the basics. Right now, two invasions are up at any given time and each invasion is available for four hours. Invasions can spawn in Azshara, Dun Morogh, Hillsbrad Foothills, Northern Barrens, Tanaris, and Westfall. To find out where they are, check your map — they’ll be marked and have a timer to say how long the invasion will last. Oh, and don’t worry if they’re in an opposing-faction city: the guards will leave you alone for the duration of the invasion. (However, you’ll want to get out quickly when the invasion’s over, otherwise you’re likely to find yourself dead.)
While the four hour cycle is pretty sluggish, it does give you the opportunity to cycle through as many alts as you’d like while you wait — and if it’s too long for your taste, Blizzard has said it will speed up as we get closer to Legion’s launch.

Heading into an invasion
Invasions take place in four stages that are fairly self-explanatory. When you enter a zone, a scenario pop up on the left-hand side of your screen will tell you which stage you’re on and list the objectives, and NPCs will shout out directions, too. If you’re not sure what to do, pop open your mini-map, which will show where the Legion’s forces are. Each invasion takes around 15 minutes, though it can take more or less time depending on how many people are in the zone working on it.
By now you probably have the idea: grab your alts, head to an invasion zone, and start swinging your sword (or mace or daggers or spells or whatever else). Then head to the next invasion zone and repeat the process. Then log out, log into another alt, and repeat the process.

Be careful!
It’s no surprise that the Burning Legion is a force to be reckoned with, and while the enemies do scale to your level, they’re still dangerous. While regular mobs shouldn’t be a problem for most classes, it’s easy to pull big groups at a time and accidentally grabbing an elite can mean your death. While there will be plenty of players out there fighting, it’s a good idea to tackle objectives as a group — either following along with the crowd or joining a party. It doesn’t matter what level the rest of your group is: mobs will still scale, so you can group with friends regardless of level. (If you don’t have friends or guildmates to group up with, the premade group finder should help you connect with others hitting up invasions.) As you might guess, groups are especially important for healers, as they tend to be extra squishy — and because you aren’t automatically put in groups during invasions, having a party will make it easier to see who needs healing.
When the boss spawns, you need to be especially wary. The bosses feel a bit overtuned, and they’re especially rough on melee classes because they do a lot of AOE and have relatively small hitboxes. Some will also leave dangerous goop on the ground that you’ll need to avoid — keep your movement cooldowns at the ready and don’t be afraid to run out of range if you’re in trouble. Remember: dead DPS does no damage!

Invasion tips
To help make the most of invasions, here are a few tidbits to remember:
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While an invasion is ongoing, the Spirit Healer works like it does in PVP, automatically rezzing you — so waiting instead of running will usually save time. However, as soon as the invasion’s done it will stop. Keep this in mind as an opposing faction player fighting a demon in town — sometimes I’ve found making a run for it when the boss is low on health has kept me alive.
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The loot crates you get will give out gear scaled to your level when you open them. There are two ways you can look at this: hoard your crates until you hit level 100 to get a set of ilvl 700+ gear immediately or open your crates right away and sell everything for gold. Items sell for level 100 prices, which can be a boon for abandoned alts on severs you don’t spend a lot of time on.
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Getting from continent to continent, especially on low-level alts, can be a real pain if you aren’t a Mage. Our advice is to get a friendly Mage to open a portal to the Shrine in Pandaria. Set your Hearthstone there and you’ll have easy access to all the major cities, making hoping back and forth a breeze. If this isn’t an option, you may want to focus on doing invasions on one continent or the other so that travel doesn’t eat up all of your invasion time.
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If you’re after Nethershard gear, you’ll want to dive into the thick of things and kill as many demons as you can. Look for skulls on your map: those are the ones that will drop shards. Looking for a Nethershard vendor when you aren’t level 100? There’s a Demon Hunter camp in each invasion zone that sells the same gear you can find in your capital cities.
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The transmog sets the Nethershard vendors sell can be learned by any class, and the Warglaives are BOA for those of you who don’t have a Demon Hunter just yet. (Sorry, collector’s edition buyers.)
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To get in on fighting the phase four boss right away, start heading back to town when phase three is between 80 and 90%.
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Bad news for tanks and casters: while you can get one-handed weapons as rewards, there are no off-hands and, for casters, no staves.
Now get out there and take the fight to the Legion level those alts!