To do this I'll start with a very short "guide" and add more detail later, so if your thirst for knowledge is satisfied you can just stop reading.
TL;DR version:
0) Never 5-star a mission you are farming.
1) Get Storm + 1 Scrapper
2) Farm 1.4.4
3) Get 30 heroes incl. Kitty Pryde
4) Farm 1.12.2
Basically that is it, as soon as you hit 12.2 you got to the by far most energy efficient CP farming spot in this game. To add a little more information to each point:
0) Never 5-star a mission you are farming.
Why not? Simple: You probably don't only want to farm energy efficient, but also minimize the invested time in repetetive farming. The moment you got your 5th star, the mission is always on your agents level. If you don't do that, you'll sooner or later outlevel the mission and just one-shot everything, reducing the invested time by a huge margin. If you want to do the mission on your level (e.g. to get more XP), you can always just do it in challenge mode, which is slightly harder than your level, but usually this doesn't matter that much.
How do you not 5-star a mission? After the last fight don't hit the "Exit Map" button, instead abort the mission and restart it.
This doesn't matter for premium missions, they are always at least your level.
1) Get Storm + 1 Scrapper
Not much to say here, you'll need those heroes to farm 1.4.4. Storm is also a decent heroes for beginners.
To get the needed CPs (unless you get them anyway by progressing through the first missions) you can either 5-star early missions or just farm 1.1.1/1.1.2.
2) Farm 1.4.4
Season 1, Chapter 4, Mission 4 (You, Foe) is not THE most efficient farming spot outside chapter 12, but very close to it. And in contrary to that one better spot, it doesn't need a ton of expensive heroes. Just keep farming it until you can Chapter 12. And just to be sure: Obviously don't fight the mini bosses on their own, this will just waste energy.
The epic boss is Magneto and he uses a pretty annoying shield, that can drag out the fight. You can ease the pain with DoTs (especially bleeds - Black Cat is a cheap hero with bleeds). If you can get your hands on any Shield Breaker (gear or ability - Havoks L1 e.g.) Magneto becomes pretty pathetic, since he only tries to renew his shield and fails. You can just kill him. However: Currently there is (to my knowledge) no Shield Breaker cheap and readily available, given that Havok costs 200 CPs and might still be the best candidate.
Another way to basically to the same to Magneto and Vapor as well are abilities with Depower. Spiral is a strong and available hero with that debuff .
3) Get 30 heroes incl. Kitty Pryde
Your ultimate target is 12.2 and to unlock chapter 12 you need to recruit 30 heroes. You also need Kitty Pryde for the deploy in 12.2, but since she is one of the cheaper heroes you will probably want to recruit her in any case.
How do you get 30 heroes as fast as possible? Well, obviously any "free" heroes (e.g. from Lockboxes) help a lot and you'll want to recruit the cheapest available heroes. Yes, Luke Cage is pretty useless, but he is cheap and one more hero to have. However, I'd suggest to still get one or two rather strong heroes very early, since they will make everything in this game a lot easier. Personally I got Wolverine and was very happy with him, since he is a pretty versatile hero and not that expensive. Captain America is also pretty strong and having him recruited will help you in farming 12.2, but is not necessary. Last but not least: You can recruit the Invisible Woman very early, since she is very cheap, not that bad early and having her will help you farming 4.4.
Should I recruit any cheap hero the moment I got enough CPs for that hero?
Not necessarily. Do a rough estimate what would be the most expensive hero to get 30 heroes if you would start recruiting. E.g.: When I started the farm towards 12.2 I could easily calculate that I would need some 48 CP heroes even after getting every single cheaper hero. In such a case you can just start with some more expensive heroes (e.g. Quicksilver, who can be pretty helpful) and get the cheaper heroes later. No need to drag Luke Cage along all the way.
4) Farm 1.12.2
Well, once you got those 30 heroes: Congratulations! That's it, you can now farm the by far most energy efficient spot for CPs. I
To give you an impression on how efficient 12.2 really is: On average this will yield about 1 CP per 10 Energy spent. 4.4 yields about half of that, as do the other efficient spots outside Chapter 12.
Further questions
I'll just give some additional information here and expand this if needed, based on questions/feedback in this thread.
I didn't get (m)any CPs in x runs! Those spots suck!
Keep calm and keep farming.
Remember: The whole thing is luck based. There can and there will be
dry runs. My personal record are six consecutive runs of 12.2 without
one single CP dropping. And there are certainly people who top that.
But in the long run this will even itself out.Won't I get more CPs if I just 5-star missions?
Short answer: No. Longer answer: I'd consider that to get Storm fast and reliable - after starting to farm 4.4 it MAY be interesting for early missions. Just consider a couple of things:
- if you need ~200+ energy to 5-star a mission, 4.4 will yield more CPs per energy.
- if you ever want to farm gear from that mission, you'll be thankful not having 5 stars. Unless you want/need to farm that on challenge mode - in that case it doesn't matter.
- if you ever need reliable CPs (e.g. to get that last hero for a timed event like a spec/covert ops thats about to run out), you'll be happy if you can still get those 5 stars in those early missions.
... but I really want to 5-star my farming mission! Why can't I?
If you really want it: Go for it. After all, this is just a guide, those are just suggestions. 5-starring will not affect the energy-efficiency of your CP-farming, unless you are playing missions (that are bad for CP-farming) just to 5-star them. Basically you are giving up the advantage of having some choices: Without 5-starring you can chose to do a quick run or a challenging run - well, more or less challenging anyway. You can also chose between very little or max XP for a run. Not having that last choice can actually result in losing energy, since it can result in an untimely level-up with automatic full energy.
However, if (e.g.) you are level 300 and don't care about the time consumption, by all means, go for it. Even though I don't think you are the target audience of this guide...
In any case: Be aware, that you can never take that back. Once you 5-starred a mission, you are stuck with it. So no whining later!
Should I buy expensive heroes to finish SOs?
That can be a difficult decision. I'd always try to get those SO heroes, since they are usually pretty nice AND having more heroes is always good. However: To finish all tasks without gold skipping you usually need some very expensive heroes, which will cost you many cheaper heroes, who would get you closer to 12.2. In light of that I'd suggest (as a rule of thumb!) to skip the CP expensive task of the SO (usually it is the Epic Boss, skippable with 25 gold) until you get to 12.2. Once you are there: Just farm the CPs while running the SO, you'll use those heroes again some way or another.
Since I got to 12.2 I was always capable of farming all needed heroes for limited time events within the time of those events. Extreme example: During SO16: Apocalypse you needed 358 CPs for the tasks (I "only" needed 335 since I already had Colossus, yay...) and 495 CPs if you wanted all horsemen heroes with one horseman alt each. Farming 12.2 I managed to farm more than those 800 CPs in less than five weeks. While also doing the SO obviously.