Experience gain has a few things that might seem complicated but they are not, and I will talk about them here.

First off to mention this:
Experience that you earn by the hero and that hero's experience boost, is given at the end of the wave to all heroes in your deck.

If you are new to DD2 and you are playing a hero that has medium sphere slots, you should do this to maximize your gain:
When you hit level 15, you should buy a 10% Experience bonus sphere and equip it.
When you hit level 20, you should buy a 20% Experience bonus sphere and replace your 10% sphere (you can use 20% spheres only after hitting level 20).
When you hit level 30, you should buy another 20% sphere and equip it in the second slot.

You can buy the spheres at the Gran Ma'ster:

Note: Spheres only give XP bonus to the hero that has them equipped and is present at the wave's end when the XP is received, but there are some tricks to that and it's more complicated than that, and I will explain it later.

You gain 35% experience boost for every level 50 hero in your deck for three heroes and that experience is cross deck. What do you mean by that?

First level 50 hero will give you a 35% bonus to experience earned by all heroes, so it's cross deck (that bonus is applied to all heroes in your deck).
Second level 50 hero will give you another 35% bonus, so you will have 75% total.
Third level 50 hero will give you another 35% bonus, so you will have 105% total.
Note: 4th level 50 hero won't give another 35% bonus, but that doesn't matter since if you have fully leveled deck you wont be farming experience, but I will mention this is anyway, so you know it :).

Now here is where the complicated and simple part come in to addon to this:
Experience that you earn by the hero and that hero's experience boost, is given at the end of the wave to all heroes in your deck.
Simple? Yes. But there is a trick that you can do to maximize your XP gain, and I will explain it in the next section.

This is primarily for players that have at least one level 50 hero, but any hero that is higher level than 30 will work.

Experience that you earn by the hero and that hero's experience boost, is given at the end of the wave to all heroes in your deck.

So you might ask: how do you maximize it based on that?
Well it's simple.
If you have a deck with three level 50 heroes and one level 1 hero, you will have 105% boost on all heroes.
So if you were to play harbinger-normal your level 1 hero would have gotten ~133k XP.
Now this is where the trick comes in:
Take one of your level 50 heroes and equip two 20% spheres, and play that hero during the waves, if you played the hero during end of the waves now your level one hero would have gotten ~155k XP.
So instead of having 105% XP at the end, you got 145%.
This works the best for heroes that can't use the experience bonus spheres like the Abyss Lord, EV2, Gunwitch.

Simple trick, but very effective.

Currently and for the last 6 months, the way has remained almost the same.
Play Harbinger's Warship on Defense Mode - Normal Difficulty.

If you have good end game gear (600 iPWR+) you will be able to complete this in under 3 minutes and get a lot of experience.

If you have friends with good DPS heroes you will be able to do it in under 2 minutes.

But even if you have much weaker gear than that, this will still be the easiest and fastest way to level up, it will just take you little longer to do it.

Now you might ask Why Harbinger?
The reason is because Harbinger gives you the best experience per wave for the time it takes to kill him.

Before the last update (Power Up and Gunwitch) you were able to play harbinger on easy difficulty where he had only ~670k HP, but after the update, lower difficulties have been "scrambled" a little and now on the lowest difficulty you can farm him, he has 2 million HP.

So it got little longer but not really harder, since the way has remained the same.

There are 4 main ways to fight him depending on your gear:

1. Barricades for occupying mobs, towers aimed on harbinger on both spawns (optional: you DPS-ing him with your hero)

2. Barricades for holding mobs while you DPS him, no towers on the side.

3. Towers aimed on harbinger, no barricades (left side towers are optional) (optional: you DPS-ing him with your hero)

4. Straight up hero DPS, no towers at all. (maybe monk serenity aura, but not necessary)
(No image needed :))

If you didn't know, easy way to figure out where harbinger spawns is to look at the "red door/hatch" thingy that he opens, it's texture is cut in 3 parts, and he spawns on the middle of the deck in front of the middle part of the "red door."

If you are playing a squishy range DPS hero, it is best that you stay away from his ship because then he won't charge at you, he will only shoot his meteors and meteor shower. When he starts the meteor shower, you can get close to him to DPS and stop him.

If he is at his cannon, shoot your cannon that is located on the opposite side of his cannon (on your ship) to damage him (cannonception :)). You can do this by pressing E or by shooting the cannon with a fast weapon.

Those should give you some insight into how to build for harbinger.
First two are for if you are finding it hard to kill him if you have weak gear.
Second two require better gear to pull off.

Don't place the towers on the deck aimed for his last wave spawn until end of wave 4 if you are using one of first 2 build, since they can get blown up.

Here are two videos that I made as a guide for option 3.

Cannon Squire (you know that you made a good guide when the youtube link has FAlL in it ): )

Abyss Lord

This NM4 tutorial might help you also, you can apply it to normal difficulty, if you are having a really hard time.

Now, this alternative is by no means replacement for Harbinger, but it's better than nothing.
Play Challenges-Normal.

That is the only way to get Daily XP bonus on maps, that I have found.
Sadly there are only 2 maps, but better than nothing.

When I did them with three level 50 character I got 230k+ XP per map (not sure about the true figure tho).

I noticed that if I didn't swap to the hero I was leveling after finishing a map, he wouldn't get exp. anymore from finishing waves. I tested this few times while leveling, and you should keep check if you experience this too.

So after you finish the map, before starting the first wave or before starting a new map, just swap to the hero you are leveling up, you don't have to do anything, after swapping to him, swap to the hero you want to use.