Edit: I'll leave this solution around as reference, but Sora's KoTH boosting method is definitely superior to this. Using it I did levels 81 through 100 in a couple of days during the 6x XP event.
Original solution to follow:
As an addition to the various solutions given, the one I've tended to go with is doing wave 1 of Security horde on insane over and over.
The basic idea is that you play the first wave, and after you beat it, you then have everyone jump into the lasers to immediately fail, and the host then picks "restart from beginning". Rinse & repeat (and repeat, and repeat, and repeat........)
On insane it's now 500 XP per wave. If you have a group of 5 who follow the tips below you can easily average well under 1 minute per wave, so with load times say about 1-1.5 mins, which is 40-60 matches an hour, which is 20000-30000 XP an hour. Not bad.
The depressing stat is that's anywhere from 220 to 350 hours to go from rank 0 to 100. Though of course if you do this during bonus XP events you can decrease those hour counts dramatically.
Now the trick (and this really only works if you have 5 people, though technically it can work with as few as 3) is to learn the 5 enemy spawn points on the map: where both grenades spawn (I'll call these points A and B), the rooms opposite the grenades (I'll call these C and D), and inside the tunnel where the lasers hiding the flamethrower are (I'll call this point E),
The key trick is to take advantage of the fact that if there is a human player on a spawn point, an enemy will not spawn there. So the strategy is as soon as the wave starts:
- have 1 player run to point E and stand there the entire match (or at least while there are more than 5 enemies remaining). THIS IS CRITICAL, this person should not leave until there are only 5 enemies remaining as otherwise enemies will start spawning in the tunnels.
- have 1 player run to point A and pick up the grenades. Similarly have another run to point B and pick up the grenades. These two players then immediately get ready to toss the grenades at points C and D, which (because of where everyone is standing) will be the only two points where enemies spawn. Again, like the player at point E, these two people should not leave their spot until there are 5 enemies remaining as otherwise they risk having enemies spawn where the grenades are.
- If you have extra players it can help to have them stand at the sandbags and play support for the players with the grenades.
- As soon as enemies appear, toss the grenades. If you time it right, the grenades will blow up all (or very nearly all) the enemies in one quick go. Then just mop up the rest.
I can't stress enough the importance of not leaving the spawn points. One of the most common issues I see is that people will run to a grenade spawn, won't see enemies spawn across from them so they leave their spot and now suddenly have enemies spawn behind them. It usually doesn't result in a fail, but it definitely does add time to the wave. Once there are 5 enemies left all enemies will be on the map and you don't have to worry about enemies spawning.
At any rate, following this I've seen waves done in as little as 23 seconds. Given that enemies first appear on the map at the 15 second mark, that means that all enemies were defeated in only 8 seconds.
If you can't get a group of 5, it is certainly possible to do wave 1 on insane by yourself, though obviously it will take much longer (I average probably about 2 to 2.5 minutes playing by myself). Strategies can vary, but usually people just camp a grenade spawn. Depending on where enemies spawn you might have a quick wave (under 1:30) or a really long wave (over 4 minutes). It really does vary considerably.
One last thing: don't underestimate public horde matches. They're only 300 XP per wave, but it's much easier to get a group of 5, and it's not nearly as boring as Security grinding. Personally I find doing an hour security grinding to be about my limit before I go nutty from the monotony, but I can play public horde for hours on end. Lately I've been doing a bit of Security grinding, then switching to public for a bit, then back to grinding, and it seems to be working pretty well for me.
Edit after TU6: with the addition of "social" matches, the former "public" mode is relatively dead, as hardly anybody is playing it anymore, instead opting for social horde. Unfortunately social horde is *VERY* poor XP as A) you either start in a match on round 1 on casual by yourself, or B) join a match on a relatively high wave on who knows what difficulty (it varies).