This achievement is best done with a second player/controller. Without this and trying it solo, the achievement comes down to pure luck, with less than a 1% chance of happening normally.
When the game starts, you'll want two possible objective rewards from the game: Defense Die and Airport. If you don't see these, quit out and restart the game.
To start, each profile picks a side and at least one continent to take. This will be your source of troops for the game. When placing capitals, place your capital on the opposite side of the map: for instance, if player 1 takes the easternmost continents, place the capital on the western side. When the game starts, each player captures the opposing capital, thus preventing the game from ending too soon. Split the map in preparation as you see fit and choose a border to contest.
The achievement profile (P1) will complete the objective for Defense Die and Airport. Place the airport in an area adjacent to your border territory. If Guaranteed Card is in the list, have the second profile (P2) complete that objective for the extra troops. P2 will build up troops right on that border, while P1 will build up troops on their airport. When P2 has six troops, they should attack P1's area with 1 troop. Instead of attacking normally, hit right so that instead of attacking with three die, P2 is only attacking with one. This gives P1 the advantage of two die vs. one, plus the airport which adds one to whatever you roll, plus the defense advantage.
If P2 succeeds, pull every troop out of that territory in the same turn. On the next turn, P1 will attack said area with their airport troops and pull their troops back to the airport to effectively reset the area. Continue doing this until you've unlocked...
Defend a minimum of 5 attackers with one defender.
The game doesn't get into overkills, but you may have seen it and its unique animations while playing. An overkill is rolling two 6s, which then destroys four troops instead of two. A super overkill is rolling three 6s, which decimates the entire force in the area, no matter how many troops they have. Additionally, for this to work, the opponent must not roll any 6s to counteract.
This is best done with three controlled players, although it's still doable with two. Choose a sizable three player map if possible.
When the game starts, check the objective rewards for: Guaranteed Card, 2 Troops, Attack Die. If all three are not available, quit the game and restart. This is where three players help, as there are more objectives and more areas on a three player map. Upon starting, split the map so that the achievement profile (P1) has half of the continents and the helper profile (P2) gets the other half. Both profiles should place their capital in the other player's territory. When the game starts, each side should take the other's capital immediately so the game won't end early. Gang up on P3 and kick them out of the game, they've served their purpose.
P1 should get Guaranteed Card and Attack Die, while P2 gets 2 Troops. This gives a steady influx of extra troops beyond the continent bonuses on both sides. P1 should place all of their troops in one border territory, whereas P2 should put a maximum of 50 troops in an area before adding more troops to an adjacent area. The goal is to have a string of 50 troop areas for P1 to eventually steamroll through. P1 should wait until they get 10 stars, then trade them in for 30 troops and an achievement.
Earn 10 stars and trade them for 30 troops.
Continue like this back and forth until P1 has a minimum of 250 troops in their territory. With a handful of restarts to get the right objectives, it should take about an hour to reach this point. At this point, attack the string of 50 troop territories. Save yourself the time by pressing X to use Fast Attack. You'll know if you got a super overkill if the defending territory drops straight down to zero. Continue attacking these territories until there are no more 50 troop territories, then use your maneuver to put your troops back in the original border territory. On the next turn, P2 should attack through the 1 troop territories and start re-establishing their 50 troop territories.
Even with all this, the achievement still comes down to luck, but a bit of patience from here is all that's needed.
Achieve 10 super overkills.
Congratulations on finishing Risk: Factions!
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