Level 40 requires a grand total of 552,400 XP.
To boost effectively, set up a private seize ground match on any map you like (I prefer Waste Management). You need a minimum of two players total, but every extra player reduces time by a _significant_ margin.
Set the score limit to 5,000, time limit to whatever turn length you like (I did 15-30 min turns). Make sure to set the respawn timer to zero ('Instant'), turn off Armour Regeneration and put Armour Strength to 'Quarter' to ensure faster dying.
IMPORTANT: Everyone on the winning team _needs_ to equip 'Rapid Deployment' as a perk (on a Tactical marine). This reduces the spawn timer at a captured point to 2 seconds (from a previous value of 7 seconds). This will roughly triple the kills gained by the booster whose turn it is. Note that I ran into a glitch whereby it did not seem to affect my respawn timer. This was fixed by rehosting.
Whichever gamer's turn it is will be alone on one team, and all other players (up to 8) will be on the other team. This team captures a point and stays there (I preferred point B on Waste Management). The booster then stands outside the capture circle and kills them in any way they wish, over and over. The killed boosters can then choose that capture point to spawn at indefinately.
The killing booster should vary his weapons to get as many weapon challenges completed as possible. Completing these challenges will gain you the lion's share of the XP you need as well as working towards the 'Keeper of the Armory' achievement.
As a reference for effectiveness, with two other boosters, I managed to get approximately 370 kills in one 25min round, unlocking numerous challenges for 104,000XP.
Acknowlegements to: WayzoftheRaven, WayzoftheHawk, PNOIDSCHIZ and montygo for boosting level 40 with me, and especially to the two former for suggesting the use of 'Rapid Deployment' to considerably speed the process.
EDIT: As mentioned by my boosting partner, make sure everyone has entered their battle code to advance past level 5. Otherwise they could lose a lot of XP!
If there is anything anyone thinks I missed, please respond in the comments and I will insert the info.