I do it the following way when I want to do serial ERs: the first time I go up doing everything in reaper (skull to your preference). I usually do everything Evening, borderlands, phiarlan carnival, chronosope, von 5-6, haunted halls, epic giant, and epic necro. If you don't do the raids, you can fill in with an explorers run + 100 kills in saltmarsh and feywild wilderness, maybe also borderlands. But this should give you more than enough xp to reach 30. I usually do an ADQ2 run every ER too: in 10 mins, a good chunk of xp. No pots.
Afterwards I usually do high reaper, or at least the legendary sagas in high reaper; If you are not interested in this, do them at least in r3-r4, which will give you a chunk of reaper xp.
With all the sagas prepared, you can do the ER, collect the sagas (if you have a pot, the better), and with a certain epic level, level up again. Doing evening sagas now in elite to take advantage of the first completions, plus some dailies should easily take you to level 28, when you can start doing legendary sagas in elite. Once at 30, finish the legendary sagas (in reaper, even if it's r1) to do the ER again. You can do this step as many times as you can endure (my tolerance for repetition is limited) doing the sagas on r1 or elite (better r1 once the first elite completion has been spent).
For the last time I level up an ER I reserve all the quests that I have been saving for this last run (none ran before): catacombs, red fens, wiz king, von 2-4, deleras, spies, the evening challenges, epic 3bc... with this after collecting the sagas easily makes another ER. Slayers is also a very quick way to do an ER. Anyways, at this point I am ready for a TR.
But this for serial ERs. If you are only going to do an ER, play reaper or elite once until cap, there is more than enough xp. You don't need to repeat if you don't want to.
