Experience point

Gaining/Earning Experience Points(XP)
There are several methods to earn experience points, in two different places: quests and Wilderness adventure areas.

Quests

 * Defeat all creatures in a single encounter in encounter areas such as Waterworks, Tangleroot Gorge, or certain special quests like Redwillow's Ruins.
 * Complete optional objectives inside a quest. Common optional objectives include slaying a boss or named creature, exploring certain areas of a map, or clearing side areas of threats.  Some optional objectives are shown from the start of a mission, while others remain hidden until triggered.  Optional objective XP is granted instantly and is not affected by main objective XP modifiers.
 * Complete all main objectives inside a quest. Some main objectives remain hidden until triggered.  This method usually confers many more experience points than the first two methods.  The main XP is bestowed upon completion of the quest and can be modified by any of the following conditions.  Changes to these conditions that occur after the main XP is awarded have no effect.
 * Difficulty setting: Completing a quest for the first time on Solo, on Normal, and on Hard difficulty each grants a +25% bonus. Completing a quest for the first time on Elite difficulty grants a +50% bonus.
 * Times completed: The first three completions have no adjustment. Completing a quest for the fourth time (i.e., the third repetition) carries a -10% penalty, with an additional -10% penalty for each further completion, for maximum penalty of -90% from the 12th completion on.  Also, beginning with the 13th completion, the maximum main XP reward is limited to 10% of its base value regardless of bonuses.  All completions at all difficulty levels count toward this penalty.  This penalty (including the 10% cap) is waived if there is a "first time" difficulty bonus, though that completion still counts toward increasing the penalty on later attempts.
 * Level of mission vs level of highest character in mission: Compare the quest's level to the level of the highest-level character to have entered the quest (even if they later left). If that character's level is below, equal to, or one level above the quest's level, there is no adjustment. If it's +2, there's a -10% penalty. +3 = -25%, +4 = -50%, +5 = -75%, +6 = NO XP. Hard difficulty adds 1 to the mission's base level for purposes of this adjustment, and Elite adds 2.
 * Death: There is a +10% Flawless Victory Bonus  if no one dies. (Incapacitation is not death unless the player releases.)
 * Re-entry: There is a +10% Persistence Bonus  if no one leaves the quest and re-enters (whether through death, recall, or exit). In addition to losing the bonus, each re-entry confers a -20% penalty, to a maximum of -90% for the fifth. Re-entry penalties do not apply on Solo difficulty, although the bonus can still be lost. This modifier does not count characters who leave and stay out.
 * Killing monsters: Killing a large portion of the monsters present can confer
 * +10% Aggresion  bonus
 * +15% Onslaught  bonus
 * +25% **Conquest**  bonus
 * Not killing monsters: On the flip side, completing a mission without slaying anything can grant
 * +8% Discreet  bonus
 * +10% Devious  bonus
 * +15% **Insidious Cunning**  bonus
 * Disarming traps: Disarming a large portion of the traps present can confer
 * +8% Tamper  bonus
 * +10% Neutralization  bonus
 * +15% **Ingenious Debilitation**  bonus
 * Finding secret doors: Finding a large portion of the secret doors present can confer
 * +8% Observance  bonus
 * +10% Perception  bonus
 * +15% **Vigilant Sight**  bonus
 * Destroying breakable objects: Destroying a large portion of the breakable objects (barrels, vases, caskets, etc.) present can confer
 * +8% Mischief  bonus
 * +10% Vandal  bonus
 * <font color='gold'>+15% **Ransack**  bonus

All bonuses are added together and can make a quest worth the time and effort to complete even if there are penalties. However, there are two cases where a quest's main XP reward will be 0 regardless of bonuses. The first is if a character enters the quest who is six or more levels above it. The second is if there are enough penalties to reduce the reward to 0 XP before bonuses are applied. Due to roundoff issues, this point is sometimes at -100% and other times at -105%.

Wilderness Areas

 * Complete Slayer objectives by killing increasing numbers of enemies anywhere in the zone. Each objective requires more kills than the last, but is also worth more XP.
 * Complete Explorer objectives by visiting locations of interest scattered throughout the zone. Each location is worth XP the first time it's visited, with a one-time bonus for visiting all of them.
 * Complete Rare Encounter objectives by killing rare named bosses. Bosses are worth XP (and a loot chest) every time one is killed, with a one-time bonus when they've all been killed once. Each rare boss has only a chance of spawning every time the area is entered, and is often replaced by an ordinary-grade placeholder enemy.

Wildernes area objectives are granted automatically the first time the area is entered. They do not have a difficulty setting. Their progress is tracked between visits, and they cannot be reset or abandoned. If any party member is above the wilderness area's maximum intended adventuring level, no member of that party can earn progress toward any area objectives. (Killing rare bosses may still award XP.)

Experience Points to Level
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The "Art" of Gaining XP
There are many opinions on the best difficulty order to complete quests to get the maximum amount of XP. Here are a few examples of various five-run methods using "The Kobolds' New Ringleader" and a party of level 1-3 characters. This is one of the most-run quests at level 2 so it makes a good example.

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Note that the commonly-suggested method of "three times on Normal, then once on Hard and once on Elite" yields close to the maximum but not quite the absolute greatest XP possible. It tends to be quicker than any method that gives more XP, though, which often makes it the most efficient plan, and it can be done solo by all players. Methods that don't start with Normal difficulty require a V.I.P. subscription (which can start on Hard) or a party.

Note also that this very rough analysis doesn't take into account how long it takes to complete each run, assumes everyone can handle Elite difficulty with no deaths, and doesn't necessarily extend to longer repetitions.