Not so much a guide, more of a clarification. You do not need 25,000 XP in your "bank" at one time. You can spend XP and the total still counts in the background.
I don't think it's so necessary to boost this. I was going through every level, completing the assault ops and buying boosters throughout. This unlocked for me by the time I got to E1, with still a third of the game left.