if you want a bit more... legit way of infinite exp without having to build a mob farm...
try to find some essence berries from Tinker's Construct, if you've 1 you're already ready to go.
then build 2 autonomous activators from Thermal Expansion for each essence berrie. put each activator on the other side of the essence berrie. one activator will remain empty, while the other will have a watering can from Extra Utilities, ofcourse don't forget to fill thise watering can. next you'll need a vaccuum hopper from OpenBlocks and some tanks of your preferred mod. place down the tank, and attach the vaccuum hopper to it, and make it so the vaccuum hopper will send the experience towards the tank. do note that the vaccuum hopper has a small range, so don't place it too far from the essence berries.
basically what happens is that one autonomous activator with the watering can will make the essence berrie quickly mature and produce it's products. the second autonomous activator will acts like a player using right click on the berrie, causing the product to pop off. the vaccuum hopper will then suck up the essence and converts it to the liquid xp from OpenBlocks which then gets transferred to the preferred tank.
I've seen this design in one of @
corjaantje his videos if you want to see a working example