This might be a bit of a long post, because there's a lot involved with how this works and rather than saying "just do this" I'm going to try and explain everything that's going on.
If a "just do this" post *is* what you want, look here: https://forum.us.forgeofempires.com/index.php?threads/higher-age-units-guide-v1-22.37109/
Ok so the process of getting these troops in Contemporary Era+ I've explained before and is relatively straightforward. Save what you can for techs, and only progress the continent map and research tree when the story tells you you have to. The reason you can't do this normally in PME or earlier is because of "hard tech blocks" that once they come up you cannot pass without progressing to the age in question. At the end of each of the CE and PME continent maps there is one of these blocks.
So how do you get by these quests? You have to stop them from appearing at all. Starting in Contemporary Era the story occasionally "breaks" depending on your actions. Story quests have prerequisites and if you've done something unexpected like fighting ahead on the map past where the next quest would be, it skips that quest - if later quests have that quest as a prerequisite, the story can disappear entirely. This can be useful to get 3 recurring quests in an age that's not the last - but in this case it's going to be used to skip a "hard tech block". By completing the last province of the CE continent and moving onto the next map while skipping the optional provinces not required before you advance the quests, it'll skip to Dictator's Death, and then the story will disappear. Presumably "King of the Desert", the CE blocker quest, requires either you to have completed another CE quest that you skipped or to control the whole map.
So your story's gone, that's all well and good - but how do you get the FE troops then? At various points, the story is eligible to return without you having completed prerequisite quests - just for being at the right spot on the continent map and not having a Story Quest. We'll call these "resume points". FE has a few of these. So we're able to rejoin the story in time to get FE troops.
Now I mentioned that Postmodern Era also has a blocker, "The End of This Chapter". I also didn't mention the story "breaking" before Contemporary Era. So... this one needs to be avoided via an anomaly in the PME story. At one point near the end of the PME story, they want to give you a choice as to whether to cover up the actions of Colonel Lang or to tell the truth about what happened, "Your Choice: Truth or Dare?". Apparently in order to make it eligible to abort, they had to flag it as a Side Quest. This allows "resume points" checking your quests to see that you don't have a Story Quest and as such let you pick up story from that point. So the core point for getting them before Postmodern Era is to simply never complete or abort "Your Choice: Truth or Dare?".
Ok so that's it for the extra complications in terms of quests... But there is another detail to deal with - you do have to somehow manage to take all these continents! To take ME through FE up to hover tanks by negotiation requires ~40k PE+ goods! That's a lot of goods to purchase, so you might want to fight as much as possible.
But for large segments of the map you'll be using units 1 or more eras out of date. Accordingly you need significant amounts of boost. The attack of units isn't much of a problem in my experience (i.e. if you have the 3 main attack buildings that should cover most of what you need to 2-shot enemy units), but given the awkward armies you're often using the defense of your attacking units is of paramount importance. So in preparation for attempting this, you may wish to focus on stacking event buildings that offer defense for attacking army. You will also need a source of a significant number of rogues.
To fight with rogues on continent map you need to move your "real unit" to the corner and keep it far enough away from enemy units that they don't pick it to attack. Continent map does not prioritize rogues over non-rogues like the AI you're used to from GvG/GBG/Neighborhood battles. But it does prioritize proximity to a large degree - so parking the real unit in the corner while you fling forward the rogues does often work. Rapid Deployment units have a tendency to occasionally blast your real unit before you can move it. And that's a big part of where the need for defense comes in.
Overall summary of process:
BA-LMA:
- Skip some unnecessary techs - not crucial to reach Hover Tanks or if your target age is Progressive Era but the more you skip the easier it'll be to reach OF troops in Industrial.
- Try to build up rogues and defense bonus for attacking army.
- Be as caught up as you can be on Story Quests before Colonial Age (you should get stuck on a quest requiring you to build Colonial buildings)
Colonial Age:
- Carefully enter it researching a minimum of techs.
- Complete Industrial Story and as much of Progressive Story as you can until you get stuck (there's some quests requiring you to build Industrial or Progressive buildings).
Industrial Age+:
- Continue carefully dealing with tech only as fast as you have to to complete story. Extreme option is to win Police Stations from events, daily challenges, or expedition so you don't have to use up techs on researching them.
- Finish PE+ME story in whatever manner you prefer.
- Only negotiate with Che Guerrero when required to take one of his provinces in PME, don't take the other yet and don't fight any sectors inside. This is necessary to receive the MG Teams that you'll be using to fight for the next while.
- Proceed with story as directed along the peninsula where you negotiated for a province from Che. Fighting or Negotiating is fine with the other leader.
- Receive 5 MG Teams (and some commandos).
- After finishing the one side and being directed by story, conquer Che's other province by whatever means you prefer (you can fight him now if you prefer)
- Finish PME story up to "Your Choice: Truth or Dare?". Do not complete or abort this quest until you are done trying to get units above your age or are in Postmodern Era.
- Complete the Contemporary Era map without advancing story while not scouting the 2 optional provinces (in the north-west) that are unnecessary to get off the map (in the north-east). Once you've scouted the first province of the TE map it is safe to advance story (possibly earlier, but if it doesn't work out you may not be able to get Hover Tanks til Contemporary Era) - it should just vanish for the time being. Complete TE map as you please.
- TE story can be resumed at "Meet Miguel Mantos" immediately after scouting Rio Roxo - and possibly other spots. Doing so may enable you to win troops useful to fight the rest of TE and FE continent with but may also require more techs. If you wish to skip the entirety of TE story, you should do the same as CE and take only what you have to and get off the map before advancing story.
- Only negotiate with Joy Danba in the first province in Future Era.
- Upon finishing your negotiations for her entire province, FE story should be ready to resume telling you to scout the next provinces.
- Follow story as directed until you reach the point where you need to scout Kalabuye and Bisoraba (the two provinces adjacent to Botuku).
- Do not touch Kalabuye until Joy Danba has given you 3 Hover Tanks.
- Continue on with story as you please. You will get 1 more Hover Tank and some assorted other troops. Tech count permitting, you may continue following story until about halfway through the OF continent map winning a great deal of additional advanced troops.
