For all the haters, here is one way to actually get the experience relatively quickly and easily. Level up details are still there at the bottom. You will need a sniper with sniper strike, preferably level 4 (total of 10 cards, 5400 scrap if you create all but the 1st), but 3 will work most of the time. Load up wave 41 on relic, Insane difficulty. Run to the fabricator, pick it up, and drop it down where it starts. As soon as the wave starts, call in a sniper strike. Wait until it is ready again, and then wait about an extra second, and call in another. Repeat until you have used all your money. The wave should be complete (or there will be 1 or 2 enemies left if you had level 3. Return to the loby, reload, and repeat.
You should be able to do this about 20 times an hour, yielding 5,000 CXP (and about 1400 credits). You will also get pretty decent MXP. About 3000-4000 for each round, so something like 80,000 MXP. This is too short a time for anyone with you to idle out, so if you can find 4 other people, they can just sit in your lobby, and get the same CXP. And if they also have sniper strike, you can do this in a rotation, and you will all have 25,000 CXP (and about 7000 credits) after 4 hours of idling, and 1 hour of work.
UPDATE: changed based on new patch evening of 10/21 which doubled CXP rate and tripled credit rate
This is not a complete guide, just some information on how you get class experience so you can decide what difficulty is going to be the best for you.
Abbreviations:
CWB = consecutive wave bonus
CXP = class xp - this is for leveling up horde classes
MXP = multiplayer XP - this is for leveling up you
CWI = consecutive wave increment, this is how much your CWB goes up by after each round
C/N/H/I = Casual/Normal/Hardcore/Insane
Tier = your set of waves (i.e. 1-9, 10-19, 20-29, 30-39, 40-49, 50. While the boss waves count as the prior tier for gameplay purposes like what bonuses the horde gets, for experience purposes, they count as the subsequent tier).
Up front, info that is unverified or i am not very confident of:
wave 50 wave xp
CWIs for Hardcore, Insane. Boss wave CWIs except for 10 on casual.
I am not sure if the CWB doesn't count the 1st wave or if it doesn't count the last wave. e.g. does running 8-10 get you the boss wave bonus or does 10-12 or do they both?
I also sometimes see odd spikes in my exp, for no understandable reason. e.g. After beating waves 1-9 on casual I would have expected to get 220 CXP, but instead got 274. I didn't do anything special, but got almost a 54 xp bonus. I try to do a good job of capturing my xp at the end of each match, but sometimes i do miss it (but I know this time I didn't). If you can help figure out what the deal is with this it would be great.
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Level class exp requirements
1 N/A
2 1,000 CXP
3 6,000 CXP
4 15,000 CXP
5 25,000 CXP
6 36,000 CXP
7 56,000 CXP
8 80,000 CXP
9 120,000 CXP
10 200,000 CXP
There are two things that give you CXP. Completing waves and CWB. They both depend on difficulty and tier. Tiers (for XP purposes) go up at the START of every boss wave. That means wave 10 is part of tier 2. They both (usually, see above) give you 40% or two fifths of your MXP as CXP. So if you earn 1000 MXP in those categories, you get 200 CXP. Ribbons don't matter, bounties don't matter for class XP.
Wave XP formulas (and amounts for all 50 waves) are:
C 105+20*tier : 8350 MXP/3340 CXP
N 80+50*tier : 11750 MXP/4700 CXP
H 60+80*tier : 15400 MXP/6160 CXP
I 45+110*tier : 19300 MXP/7720 CXP
CWB is based on how many waves you have completed since failing/restarting. Each round (after the 1st), you earn an amount based on how many waves you have beaten without dying. The exact amount varies based on difficulty and tier. Normal 1-9 is 5 per wave. So if you complete 1-2 w/o dying you get 5. 1-3 is 5+10=15. 1-4 is 5+10+15=30 and so on and so on. You get another bump for each full 10 waves (this is an additional 5 MXP on casual 1-10, but keep in mind that 5 carries over until you die, so 200 for this one on a perfect run, and there are presumably other, larger bumps after 11-20, 21-30, etc)
Numeric details are at the bottom, but your CWB continues to accumulate after each round, including boss waves, and only resets to 0 if you fail a wave. This means if you can manage to complete every wave without ever dying your CWB will give you something in the neighborhood of 960 additonal CXP on casual. (That's about a 60% increase over just the waves.)
For going 1-50 w/o dying you should get at least (on top of the normal wave XP):
C 2307 CXP
N 3774 CXP
H 6078 CXP
I 13900 CXP
CWB goes up by a fixed amount every wave you beat, based on the tier.
Credit return was also increased by the patch. It was previously 1 for each 25 MXP you earned as part of the wave bonus. CWB credit rewards were negligible, less than a quarter of that. This patch roughly tripled the wave part, but seems to have left the CWB part as is.
For a full run of each difficulty you should get about:
C 1028
N 1446
H 1895
I 2375
Thanks to K0lding for helping get a lot of these numbers together.