All is in the title, actually the only mean to have xp is by grinding mobs or with the autoclicker-carrots setup that I told you about.

However I think that having to use an autoclicker is like cheating I don't want to do it.
For the mob farm, the problem is the lag generated by mobs, because you don't have any mean to automate it properly (no fan, no vector plates, no conveyor belt) and even using water to push mobs is too slow when using a very big farm (and to reach lvl 256 everywhere you need a really VERY big farm).
And even the most optimised mob farm will generate some lag, and when you play on a server and about 20 players start to make their own mob farm bigger and bigger as the xp needed increase, the tps will just decrease and make the game unplayable (on the server I play the admins are forced to give us free xp because of this).

To sum up, mob farm is really efficient to get xp in singleplayer or in a little server (with few players in the same team) but it's the worst idea ever on a big multiplayer server.

So thats why I ask you to add another mean to get xp, perhaps an xp seed that give solidified experience ? Or using the EMC essence to make liquid xp (and the better the seed tier, the more xp you gain)
Setting an EMC value for the solidified xp can also work but I really will not like this (as xp will be free if the value is too small or useless if the value is too high).

Well thanks for reading this issue (sorry for my bad english I use google traduction most of time), I really love this pack, now I try to automate the tier 29 EMC seed (all before are already done) and I don't think to leave this pack before a moment.

At one point (back before I moved the pack to skyblock) I had experience seeds. The issue with them is mainly that given I only had one tier xp seeds, and it turned out using copper/osmium as a material in a pick was better for xp and then just strip mining with veinminer.

Though I do like the idea of having a way to convert different EMC essence tiers to experience. (Though that certainly won't be for the initially release version).

One thing you could try, which I have done in the past (even though it was on a smaller server) is using powered spawners from Ender IO to spawn mobs. And then push them against a wall/into a pit using Interdiction Torches from ProjectE. In that 2 deep hole, you then have an atomic reconstructor pointing at their feet with a lens of the killer attached. Put a vacuum chest, and xp vacuum nearby and as long as you have the reconstructor on a short timer it is passive and (at least in my experience) pretty lag free xp.