For single player, the factor is 1 (equivalent of a 4 ppl multiplayer game).

Game and Map Difficulties
Game difficulty (Easy/Normal/Hard) scales your damage as well as monster damage. Because experience gain is generally based on damage, you will get more experience on Easy difficulty over a certain amount of time. The game difficulty doesn't change the amount of experience a creature is worth; You just gain it faster. Here's a table of damage scale changes:

Map difficulty (Regular/Veteran/Elite) doesn't seem to scale monster attributes per se. Instead, the monsters populating the higher map difficulties have higher inherent health an experience values (individually redefined) apparently.

Combat Leveling
Combat leveling (not "combat magic leveling") is the term used for gaining experience by inflicting damage to enemies. Every time you inflict damage on a monster, you receive a portion of it's experience value.

points_awarded = (damage / max_hitpoints) * exp_value

Where,

Given this, one can probably conclude that damage per hit doesn't affect the total amount of experience you gain. You don't get more experience for dealing more damage per hit, although you get more experience faster (assuming same weapon speed). For example, if you do 150 damage to a monster that has a max of 1500 life and 1000 exp value, then you will get ((150/1500) * 1000 * 10 hits) = 1000 xp for killing it. Say you do 1500 damage to it in one hit (hypothetical, of course). You'll get ((1500/1500) * 1000 * 1 hit) = 1000 xp for killing it on the first hit.

Damage inflicted can be reduced by the target's defense and by other various factors, which is why you oftentimes don't seem to get as much experience from a boss or mini-boss in the same amount of time that you took to kill all its henchmen. Additionally, there are experience limiting factors in formulas.gas (0.10 for the first level, and 0.025 for subsequent levels). I personally don't know (yet) how they affect experience gain, but keep in mind that this may be yet another instance of where experience gain isn't a straightforward system in this game.

Passive Leveling
There are a few other ways to get experience, but they usually aren't even in the same ballpark as combat leveling, in terms of the rate or amount of increase.

1. Non-offensive and Indirectly-offensive Spells: spells that don't do damage, such as Major Heal, usually give experience for just being cast.
2. Damage Reflection: damage reflected items will award experience to whatever skill you were using when damage inflicted to you was reciprocated. This applies to spells that increase damage reciprication, but see the note below.
3. Reactive Armors: armor spells that retaliate against inflicted damage give the caster (who cast it last on the target) experience. So you can cast it on other party members to personally benefit from their benefits :)

You also get experience for completing quests, but that is usually so small that you can't really tell the difference.

Experience Distribution
Every time you get X experience points, by using skill Y, you get X points added to skill Y. Additionally, you get some fraction of X added to STR, another fraction of X added to DEX, and yet another fraction of X added to INT. 

Level Increase Influences
You notice that using Melee skill will increase your Strength (henceforth referred to as STR) faster than using the other three skills. Similarly exercising the Range skill will increase Dexterity (DEX) faster, and both Magic schools level intelligence (INT) faster. That's because each skill has at set of influence values for each of the statistics (STR/DEX/INT). These influence values describe the influence that each skill has on increasing each statistic. In other words, it helps to describe what portion of experience earned using a certain skill goes into the experience count for each of the statistics.

You can find the influences listed in the game's formulas.gas file, but here it is in a table as reference:

Uber Level
Your Uber level is the character level you see displayed when entering a game's staging area (join configuration screen). It is essentially the sum of all the experience points you've gained in the game translated into a character level. A general rule of thumb you can use to compute it for yourself is to add up the experience values of all your skills, and use that sum's skill to look up the skill level corresponding to whichever experience requirement it satisfies in the experience table.