Co-op Leveling

Completing Co-op Missions grants experience points either to the commander being played (up to level 15) or towards the players Mastery Levels. These points do not contribute to the player’s Multiplayer Levels, but do contribute towards War Chest rewards.

Gaining Experience
Successfully completing a Co-Op mission grants 20,000 experience, plus up to 2,000 additional points for completing the level's bonus objectives. On top of this there are multipliers for difficulty and map selection:

Percentage bonuses are always calculated against the base reward before any other bonuses are applied, so a mission completed on Brutal difficulty on a random map with all bonus objectives will be 22,000 + 22,000 (Brutal bonus) + 5,500 (Random map bonus) = 49,500.

Co-op play also grants a daily experience bonus of 10,000 for your first win of the day.

Mutation Bounties
Each weekly mutatuion grants a bounty based on the highest difficulty you have beaten the mission that week. The maximum possible weekly bounty is 185,000 experience points.

Leveling a Commander to 15
For each commander, the progression is as follows:

Power Leveling
Assuming you are queueing for a random partner, the quickest way to get a new hero to level 15 is to grind Hard difficulty on the quickest map, Rifts to Korhal.

Including bonus objective this offers 33,000 XP per mission, or at most 32 missions to reach maximum level. On Hard with a random partner the level takes between 15-20 minutes to clear, which means between 8 and 10 hours of play to level a single commander from 1-15.

For casual gamers spreading this effort over several days, the 10,000xp first win of the day bonus will reduce the total time necessary. For example, a player leveling a single commander over the course of a month will accumulate 300,000 experience from their daily bonuses, reducing the required number of completed missions by 9, or 2-3 hours time played.