In order to achieve this, I started a new, Sandbox campaign, with unlimited funds, 'modern' technology-age, 50 starting people, and every other setting on 'very low' or 'none' EXCEPT political difficulty, which I set to 'very high.' (I repeated the achievement on 'very low,' and had exactly the same end result, but it simply took a little longer) -- I chose "Ultimo's Island," though I doubt choice of island matters, at all.
1) The first thing I did was quick-build a 'Military Fort,' (with colonial soldiers) -- DO NOT upgrade the fort, as this will be the source of the coup, and you'll want your palace death squad troops to defeat them, easily.
Here were my next steps:
2) Lower pay in all buildings to level 1 pay, except the palace, which I lowered to level 2 pay.
3) I happened to have a 'Foreman' in my dynasty, so put him in charge of the Military Fort (to lower job satisfaction more), and put a 'General' from my dynasty in charge of the Palace. Additionally, I upgraded my Palace troops to 'Personal Death Squad.'
4) I fired everyone in the Construction Office, and locked-out that office from re-hiring anyone.
5) Once the Military Fort had a full contingent of troops, I looked at each individual troop's Approval Rating of me (click on fort,
once to troop page,
to a specific troop, and click
to look at that soldier's approval of you), and assassinated any soldier with an Approval Rating
above 25 -- this helps to make the Fort's soldiers more disgruntled.
6) I then put the game on high-speed, to await some sort of unrest to happen, and to let my soldiers' loyalty % decrease (check this with
+
, and scroll down with
to the 'Political' tab). If any peaceful protests happen during this time, "Send in the Army" to quash the protest.
7) Eventually, I lowered the Palace pay rate down to the lowest level, as well. By the time soldier loyalty % hit 15%, you should have "High," or even "Very High" unrest levels. Continue on, as it's just a waiting game, at this point.
8) When the coup happens, SAVE YOUR GAME immediately. Since the Military Fort has ONLY soldiers who have VERY low opinions of you, the coup will have happened here, so you'll only have a squad of Colonial Soldiers to deal with, using your 'Personal Death Squad' palace troops.
Once your palace troops have dealt with the fort soldiers, the achievement will pop. If, by some miracle, the colonial soldiers (with 41 attack rating) beat your palace troops (with 62 attack rating), then re-load your save and let the fight happen, again -- but, this shouldn't be a problem.