Just spam the mission 'observe from above' after you dump your points into the long bake missions that cost 4 energy. 'Observe from above' cost 5 energy, gives 28xp, and has a 10 minute cool down. If you energy bar is at a zero state it takes 25 minutes to re-play the mission...so you just need to log in every 30 to 2hours and 5 minutes to play it. On a 25 hour cycle the mission can be played 60 times and will yield a 1680 xp, also keep in mind that it has a bonus xp proc as well.

If you simply need to dump points any of the 7energy/35xp instant missions are good as well. If you play those from a bar regenerating from its zero state, at the 35 minute replay rate you can play the mission 42 times in a 25 hour period, it will yield the second highest xp payout at 1470. The benefit of these mission are there is no cool down. So you can spam down your energy bar to zero if you think you won't be able to log in within the 2 hour and 5 minute mark.

The only situation were something yield a higher payout then 'observe from above' in the same amount of time is within the first hour if you spam down a 7 energy/35 xp mission from 25 energy. You'll get a 105 energy off of the initial spend down, then it will take 15 minutes to refresh the energy it takes to play it again...then another 35 minutes to re-play, which will yield 175xp within that first hour. Observe from above only wields 140. Although, if you are playing on a 25 hour cycle, the 6 long bake missions that cost 4 energy, will yield 200 xp, and you can start the playing the 'observe from above' cycle within 20 minutes of that initial spend down.

'Observe from above' also gives the highest gold payout, with the chance to proc bonus both xp and gold. I highly suggest spamming 'observe from above' unless you have a much better reason.