I've got one save that is level 6,872. Once you learn the mechanics of the game, it's not very hard to achieve. You should never have a game that lasts more than an hour and a half.
Search the forums and learn how to mana farm. Also search the web for maps that show how much mana is available on each level, and what the max exp. available is.
Start out on the levels with the most mana available in mana shards. They will give you a head start by allowing you to create powerful gems fairly early on and get you pretty far in survival mode.
Once you've reached a high enough level that you can create some very high damage gems, look for the levels that offer the most exp.
By now you should be good at mana farming and have a good setup.
The way you're going to max out the exp on the level is by getting all the amulets available. Here's the trick - DON'T RUSH. there will be plenty of time for all the amulets once you're set up with a level 30+ gem. You don't need to call waves early. That does you no good in endurance mode. You don't even need to anger the waves until you're ready. When you do anger the waves, ONLY CALL 900 MONSTERS IN YOUR ANGER. That's it. You don't get any more exp for calling any more monsters than 900 in order to get the amulet.
You don't need to play forever either. Once you've gotten all the amulets available for that level (there's a tab at the top that will show you what you've earned/what is left) you can hurry and end it. If you've got a level 32+ gem enhanced by level 28+ gems, it will zap anything as soon as it shows its face. Go ahead and launch all the remaining waves. After level 1337 it will end and if you get the "Do not let any monster touch your orb" amulet, you 'll max out the points for that level. Really, once you launch all the rest of the waves on high speed, it shouldn't go for any longer than 15 min.
I think the level with the most exp is 118 million points. The secret corner levels are great for both mana farming, and exp gains. Once you have the techniques down, it's easy - and as I said before, no level should last for more than an hour and a half, but once your level is high enough, you can crank out a survival round in a half an hour or so.